SZ 50 EP Question for Grant

[maxnix]

I notice at http://www.tirerack.com/tires/...50+EP the 245/45ZR17 SL are rated for a 1521 lb. Maximum load while the 255/45ZR17 SL are rated only for 1389 lb. Maximum load. Meanwhile, the overall daimeter is given as 26.1" for both sizes. Is this data correct?

For the wheel selections presented for a G50, I notice that in 17" there are no 8.5" rim widths available and that 9" rim widths are indicated for rear use only. In larger diameters, the 8.5" rim width is available. Why not in 17", and are these actual physical fitments, or calculated from manufacturers data? I ask because on this board, it is well known that the rear wheel for the 300ZXTT 16"x8.5" is an ideal fit for the G50.

Also, the site suggests a 235/45 17" for the front and a 255/40 17" for the rear with the SSR GT-1 17"x8" Front and 17"x9" rear. Unfortunately, both of these tires do not meet the minimum static load requirements of the vehicle. Neither do they conform to the 26" stock rolling diameter. Is this an oversight, or does tirerack advocate undertiring a heavy car?

It would be helpful if wheel strength and weight were available at the site also. Cosmetics are nice, but information about wheel specifications is more important.

By the way, I am a satified Tire Rack customer. It is a top notch organization with great customer service. I am just a little leery of the data presented and how it is verified.

[szh]

By the way, I am a satified Tire Rack customer. It is a top notch organization with great customer service.

I second that comment!

Z

[Grant@tirerack]

I'm assuming this is on a '95 model from your signature. The stock tire size of 215/65-15 is rated for 1378 lbs. when inflated to the stock 29psi.. Both tires are fine on load for this car. On some sizes you need to run slightly higher psi than stock (which was very low to begin with). The stock tire size can handle up to 1510 lbs. at 35psi so there is a lot more load capacity available with that tire size than what you need for the car according to Infinity. 1378 is the load rating listed by Infinity for the car not 1500+ lbs. We just had a very extended thread on another car about this. Both of these tire sizes are either right atthe stock 26" or within 3/10" of the stock O.D. so the rolling diam. is not enough to affect anything on the car. As for the wheels, we sell only O.E. quality wheels or better. There is no standard for wheel strength you can use to qualify wheels other than what is listed as O.E.. In the U.S. market, you could cast a wheel in your garage at home and sell it on the internet. The minimum standard we set for all of our wheels is the same as what is used for T.U.V. since we sell a lot of wheels overseas. There is no such standard in the U.S. and T.U.V. is generally seen as the highest out there. We sell the sizes we have available that meet that standard and that we know based on actual test fitments on the car will fit without modification. Being mail order we tend to be more conservative because the wheel and tire has to fit the first time when the customer puts it on the car. There are not as many wide wheels for this application in the 17" range simply because most vehicles with this wide of a rim would require a different offset than what is typically used on the car. Newer cars with larger brakes and more fender clearance will be able to handle that type of fitment easily which is why you will see more of them in an 18"+ size. If they made it, we would sell it. I can check on the weights if you let me know which models you are interested in. Just drop me a line or call me direct.

[PalmerWMD]

That's pretty impressive, I didnt realize all tirerack wheels pass TUV standards.

Fred...:thumbup

[maxnix]

I'm assuming this is on a '95 model from your signature. The stock tire size of 215/65-15 is rated for 1378 lbs. when inflated to the stock 29psi..

Which is why no knowledgeable Q45 owner runs them at that low of a pressure. The static load with half a tank of fuel and just the driver is 1200 lbs. on each front tire, so dynamic cornering and braking loads are even greater. It's still a dicey proposition to mount tires of less load carrying capacity than OEM because OEM specifications are the minimum specification that the manufacturer believes will function on the car. This is doubly true when the car weighs two tons like the G50.

Which brings up a question I have always had: How can most tire retailers claim OEM suggested pressures are sufficient when the maximum loading occurs at higher pressures for newer optional tires than for OEM tires? 29 lbs. psi on a tire which achieves maximum load capacity at 36 psi is a lot different than in a tire that achieves it maximum load capacity at 51 psi.

Also, is the load carrying capacity of a tire roughly linear across it's operational range (say OEM minimum to load capacity maximum pressure)? For the OEM tire cited above, (29/36) X 1521 = 1225.1 lbs. (the static load!) Quite a bit less than the 1378 lbs. @ 29 lbs. psi cited above.

Would you comment on the decrease of load rating of the Firestone SZ 50 EP I mentioned before? Is this a misprint or documented by testing? And I don't really understand why TR doesn't specify the correct 255/45 - 17 tire for the G50 Q45 when it is availble at essentially the same price.

As far as the wheels which TR carries, unless I am wrong, only the SSR and some BBS seem to be forged. Centerline does list a load carrying capacity on each wheel they sell, but they do not state how they arrive at that figure. TUV is a uniform standard, but I wonder if they test on a lighter car, or on a laboratory jig? Surely they must accomodate the big Mercedes and BMWs in their procedures.

Does TR test each wheel for OEM specifications for run-out and balance before mounting the tires when they are purchased as a package?

Thanks for sharing your knowledge.__________________Brian1995 Q45 & Q45t

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[Grant@tirerack]

You seem to be confusing load capacity and load rating. The stock tires have a load capacity of 1510 lbs at the maximum inflation pressure the tire is designed to handle. 1378 is the O.E. load capacity listed by Infinity for the car. The load rating of any tire is based on what is called the Load Index. This is listed as part of the service description on the tire sidewall. If you look at the tire sidewall on the SZ50 EP in the 245/45-17 size it is a 95 load index tire. At 35 psi cold pressure, the load limit for this tire is 1521 lbs per tire. The stock tire load capacity at 29psi cold pressue is 1378 lbs. That's 143 lbs more capacity per tire than the knowledgeable Q45 designers and engineers tested and determined was correct for the car. I like to think they know a little more than any of us about the static and dynamic load limits of the car they designed and tested . You could run these as low as 32 and still exceed that load rating with this tire. The 255/45-17 only has a load index of 92. This is due to the difference in sidewall construction and air chamber size. This means that at 35psi cold pressure the tire can only handle 1389 lbs per tire. Perhaps that is why we don't list this as 'the correct tire' for the G50 You can use this size but you would have to increase the air pressure to at lest 35 psi. to maintain the stock load rating. You would need at least an 8" wide rim with that. A 9" is ideal for handling and stability but they will only fit in the rear. That is why we only list it as a rear size. With an 8.5" up front that would be a lot tighter fit than we are comfortable with and if, heaven forbid, a customer wants to lower his car or add coil overs, there just isn't that much room to work with. We don't consider any fitment proper if there is any chance of even 'a little rubbing'. The 300ZXTT wheels may fit fine up front. We have never test fit them here since we don't sell the wheels and don't have any reason to cross fit O.E. wheels like that. If they work, that's great! As for the wheels we sell, SSR, BBS, and OZ Racing all make forged wheels (OZ being the multipiece wheel centers) of some sort. The vast majority of BBS wheels are still counter pressure cast as are most of the OZ Racing one piece wheels. All of the wheels we handle are made to the same standards these vendors use for the O.E. wheels they produce. TUV is not exactly a uniform standard. It is very vehicle specific and takes into consideration vehicle weight, load, and other safety factors. This is the most detailed testing done world wide and is done both on the vehicles and in the lab. The Germans are very *$#& when it comes to safety and rightly so. If you are caught with non approved wheels on your car over there, it can be impounded on the spot. One of the main reasons we mount and balance before we ship them is to check everything out for runout and balance on the Hunter machines. That is the only way to know for sure they are good to go when they get there.

[Q45tech]

The problem is tires are only required to have a 12% safety factor based on static weight. Consumer advocates have been trying for 15 years to get this upped to 15%.........you wouldn't think that would be a big deal but the opposition is fearce.

It is assummed that a normal use would not include more than occasional braking or turning where the weight transfer can easily exceed the maximum load rating on the tire, in street operation.

Agressive drivers are assummed to be smart enough to buy tires appropriate to their usage requirements......over and above the minimal specifications chosen by factory.

Darwin takes care of the rest!

On a heavy car designed with aggresive camber curves one cannot assume all tire even those rated the same are the same!

They might have similar capacity brand new, but degrade at faster rates is the best you can hope for. I pray that the tires I buy are good enough at 10,000 miles. Other than bidders for factory tire contracts, tires are never tested on individual cars, they are tested in the 72F lab against a wheel.............which cannot be correlated to an individual car in anything but going straight.

THEY NEVER TEST TIRES WITH THE CAMBER AND LOAD in PLACE SIMULATANEOUSLY! THEY NEVER TEST TIRES with any added camber gain in turns unless the factory demands it.

[maxnix]

[quote=" Grant@tirerack You seem to be confusing load capacity and load rating. The stock tires have a load capacity of 1510 lbs at the maximum inflation pressure the tire is designed to handle. 1378 is the O.E. load capacity listed by Infinity for the car. The load rating of any tire is based on what is called the Load Index. This is listed as part of the service description on the tire sidewall. If you look at the tire sidewall on the SZ50 EP in the 245/45-17 size it is a 95 load index tire. At 35 psi cold pressure, the load limit for this tire is 1521 lbs per tire. The stock tire load capacity at 29psi cold pressue is 1378 lbs. That's 143 lbs more capacity per tire than the knowledgeable Q45 designers and engineers tested and determined was correct for the car. ... . You could run these as low as 32 and still exceed that load rating with this tire. The 255/45-17 only has a load index of 92. This is due to the difference in sidewall construction and air chamber size. This means that at 35psi cold pressure the tire can only handle 1389 lbs per tire. [/quote]

Grant, thanks for the reply. More questions arise, and some original ones have gone unanswered. To wit:

So, the TR recommended 225/50-17 in Michelin A/S (certainly a strong tire carcass) has a load capacity of 94, as does the recommended optional rear fitment of 255/40-17. This is still less then the 95 required by Infiniti. What good are these numbers if they are not an accurate index to determine minimum load capacity requirements?

I guess I don't understand why the SZ 50 EP chart says the maximum load carrying capacity of 1521 for the SZ 50 EP tire occurs at 44psi, but you say it occurs at only 35 psi. So from your calculation, the relation of actual pressure to maximum load carrying pressure is not linear across the normal operating range (OEM minimum psi to tire maximum load carrying capacity psi) of tire pressures?

So the 255/45-17 SZ 50 EP Firestone has a different construction sidewall construction than all the other SZ 50 EP tires? Interesting. I'll have to read the sidewall when I can view one. Michelin nor any other tire maker who makes a tire in this size downgrades this size. It's wierd there is no load rating listed for the Firestones. Are they not on the tire? I thought it was legally required to be displayed there.

I assume since the G50 was never available in Europe, the TUV approved wheels TR recommends for it are styles TUV approved for Mercedes S class, BMW 7er, and Audi 8 series cars?

I will narrow my wheel candidates down to the ones you cite as forged.__________________Brian1995 Q45 & Q45t

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[Q45tech]

We all see the not so pretty results of weak tires on our Q! The inside edge wears twice as fast as the center and many tires go out of balance weekly as the internal stresses destroy the construction causing belt shifts and thousands of microscopic filaments go bong further weakening the tire. They usually don't fail they just wear fast and irregularly. They vibrate and hop further degrading the driving experience.

The tire blocks heel and toe feather from insufficient rigidity as the tire squirms down the road.

A few tires perform decently and the bulk do not, when all are rated at 95V.

If I was going for less than Michelin Strength, I would at least up the load index to 97, 98 or 100. To hopefully gain some longevity.

[AZhitman]

I just ran into this issue with my 255/50 Falkens.

They are listed as a 99W load rating, but mysteriously, their load capacity is listed as SIGNIFICANTLY less than their 245/50 tire.

So what gives? Do I need to pull these off and return them, since they're not sufficient for my car's weight, or do I have an exceptional margin given the 99W designation?

[szh]

If rememer correctly, the Falken web site lists many of the Ziex sizes as being "REINFORCED" for the extra load capacity.

Z

[Grant@tirerack]

Sorry for the delay. I was out road testing tires this afternoon (I love my job ).

The Q45 requires at least a 95 load index tire if you run it at the stock 29 psi.. The load index refers to the tire, not the car and that's the minimum for most normal driving. That is not in doubt. To say the car requires at least a 95 load index tire is not exactly correct. That only holds true if you intend to run the stock pressure. If you drive in even a slightly spirited manner you will be tearing up tires at 29psi. As with any vehicle, you have to adjust the pressure up to match your driving style. That goes without saying. You can also run a 225/50-17 load index 94 tire as long as you run the correct psi to match the load capacity required for the car. Having the alignment set to the most positive limit of the factory settings will help somewhat (check with the ship aligning the car for the specs)but even that is going to be a bear on the inner tread. make sure the toe in is correct as well.

When it comes to the load index numbers and maximum load, it's important to remember the maximum load ratings listed are always at 35 psi, not the maximum pressure the tire is rated at. I don't think I made it very clear. That is the industry standard method of rating load index. Many newer tires have been constructed to handle higher pressures (like the SZ50 EP in the 245/45-17 size). Most of the tire companies have been bumping up the max. pressure newer tires can handle (even on standard passenger tires). This is due in part to the new federal regulations (search The Tread Act on Google for details). They are expanding the safety margin Q45Tech was talking about before it becomes required. If the tire has a maximum pressure of 44 or even 50 it gives you the ability to increase the pressure to acheive lower rolling resistance and better resistance to deflection at higher speeds. The whole load index system is in the process of being revamped and debated by the industry right now and the tire makers have seen this on the horizon for some time.

It's not that the tire makers are downgrading tires, it's that they are beefing up the newer tires so they can handle higher air pressures to increase that safety margin. You will start seeing more tires in the same sizes with higher load index ratings in the next year or so. They can do this by increasing the sidewall ply thickness and other construction methods to reduce deflection in the tread and sidewall when under load. A lot of tires are already being made to handle the higher pressures but the molds have not been updated for the sidewall markings. The tire industry is in a mad rush to up those pressure ratings to increase load and capacity. I am still looking for a website with the index charts so I can post a link. It's much easier to understand if you can see how the tables are laid out.

Even if a vehicle is not available in Europe, they can still determine the specs required for a wheel based on the car's weight, suspension, and other factors. If you do a Google search on T.U.V., wheels I'm sure they have all the test procedures explained in depth. These guys are very picky in a good way.:pface

[Grant@tirerack]

As luck would have it, I don't list a 99 Load Index on any chart for a 255/50-17 tire. Check with Falcon directly to see if they can break it down by pressure for you. They should have the specs on that. Like I said earlier, these higher psi. rated tires are starting to hit the market in advance of the new standards for safety and increased load ratings.

[szh]

Hi, AZ.

The PDF of the Falken ZIEX specs from their web site shows the 255/50-16 as a 99Y tire with a max load of 1687 lbs. You should be okay! Note that the on-line FLASH imaged tire catalog has it as 99Y with a max load of 1201R (where the R means reinforced).

I suspect that the PDF is accurate - it looks like it came from a brochure. But, to be safest, I'd suggest calling them!

Z

[maxnix]

Thanks Grant. I never realized that all tires achieved maximum load carrying capacity at 35 psi, reagardless whether maximum pressure was 36, 44, or 51 psi. I'll re-read my sidewalls again, because I always thought it read something like "1521 lbs. Maximum load @ 44 psi."

So the pressure relationship is not linear, and probably cannot be determined without a lot of destructive testing at various pressures and loads, which no one is willing to do.

Hope to hear some more comments from you soon about this non-transparent subject.__________________Brian1995 Q45 & Q45t

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[Grant@tirerack]

The rating system in place at this time only requires testing up to 35psi so all the load index ratings top out at that pressure. The DOT and other governing bodies are in the very heated process of revamping all of these standards with the tire makers. I have been looking for over a week now online trying to find a site with these load index tables. It's a very touchy subject with the tire makers now which is probably why I'm having a hard time tracking it down in a 'public' fourm. As early as a year ago, most of the major tire company web sites had those posted on their web pages. I'll keep looking.

[Q45tech]

The 35 psi relationship is only true for older multiply sidewalls where the maximum inflation pressure was 36 psi.

The destructive testing proceedures only tested to 35 psi and never exceeded 100F tread temperature............180F is the normal optimum temperature for summer tires to achieve maximum grip.

Federal tests are for passenger cars driving straight on the highway at legal speeds. They assume NO CAMBER and perfect toe thus no side or non perfect loads.

http://www-nrd.nhtsa.dot.gov/p...2.pdfNice that between 9 and 17% of tires failed these simple test.

http://www-nrd.nhtsa.dot.gov/v...s.pdf

"The proposed endurance test will be appropriately more difficult to pass, largely due to the increase in test speed from 50 to 75 mph. We support this change. Other changes from the current tests, including higher ambient temperature (+ 2°C), higher loads (+5% and +10% for passenger tires), and a longer test duration (+6 hours more than FMVSS 109), collectively toughen the requirements."http://www.consumersunion.org/products/tires.htm

""Handling is the first criterion (OEMs) use to accept or reject a tire for a new vehicle," says Phil Pacsi, product information and education specialist at Bridgestone/ Firestone Inc.

Engineers working on original-equipment (OE) rubber also concentrate on fuel economy and rolling resistance, noise, wet handling and ride comfort. Wear is almost a secondary automaker requirement these days, say tire manufacturers.""WASHINGTON (Aug. 7, 2003) — A U.S. Appeals Court in New York has overturned the tire pressure monitoring requirement mandated by the National Highway Traffic Safety Administration (NHTSA), declaring it “contrary to law and arbitrary and capricious."http://www.nhtsa.dot.gov/cars/....htmlh ... /tires.htm

http://www-nrd.nhtsa.dot.gov/v...e.pdfWet and dry sliding and peak friction vs inflation pressure

[AZhitman]

Hi, AZ.

The PDF of the Falken ZIEX specs from their web site shows the 255/50-16 as a 99Y tire with a max load of 1687 lbs. You should be okay! Note that the on-line FLASH imaged tire catalog has it as 99Y with a max load of 1201R (where the R means reinforced).

I suspect that the PDF is accurate - it looks like it came from a brochure. But, to be safest, I'd suggest calling them!

Z

That's what I was seeing then - I never downloaded the PDF. I did email them and alert them to the difference.

All I can say is I have over 1000 miles on them now, and the little nubbies (on the tread face) haven't even completely worn away yet.

[Grant@tirerack]

"The 35 psi relationship is only true for older multiply sidewalls where the maximum inflation pressure was 36 psi."

That is ture but only when looking at the newer extra load (XL) tires and they will be marked as such on the sidewall. Those have their load index rated at 41psi. Standard load tires (which is what we have been looking at) all have the Load Index rated at 35 or 36 psi. Not to confuse things but to get very specific, P-Metric tires (size numbers starting with P ) are rated at 35 psi. Euro-Metric tires (without the P in front of the size) are rated at 36 psi..

The links provided are a good heads up on the ongoing debate on this topic and the way tires are tested (and not tested) now. Thanks for posting those ! :thumbup Q45Tech, do you know of any web sites with the pressure/load index tables listed by tire size? For the life of me, I can't find them anywhere on the web. I can find plenty of charts for truck tires but squat on standard load tires.

[maxnix]

Thank you gentelmen for your well considered explanations. I now know I didn't know squat about this subject. A good explanation of Grant's load capacity vs. pressure rating can be found at (surprise!):http://www.tirerack.com/tires/...range

The Falken Ziex ZE-512 speicifications at http://www.discounttiredirect....28780seem to indicate a 99 load rating.

Now I wonder why some Michelins, the identical tire in the identical size, one may have a W speed rating and the other a Y speed rating. What are the actual differences? Compound only, or structural with regard to the carcass? __________________Brian1995 Q45 & Q45t

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[Grant@tirerack]

The speed rating has more to do with the internal construction of the tire. In theory, the Y rated tire will have more resistance to deflection due to heavier duty support layers/materials in the belt and sidewall areas.

[szh]

Many years ago, tire manufacturers would list the internal materials used in each tire. So, it was pretty easy to see the internal construction differences between two ratings in the same size for the same tire.

Not common now, though. Plus, there are far more sizes available - perhaps too many for each model!?! Look at the Kumho Ecsta 712 spec sheet at Tire Rack for an example of what I am talking about.

Z

[Q45tech]

In one of the links, Michelin had statistcs that showed that almost no [properly loaded/inflated] tires failed in less than 2 years or 20,000 miles. The curve rose exponentially after that!

They were attempting to create a usable accelerated aging proceedure so that tires could be tested quickly - in 30 days or less.

Pirelli use to have wonderful charts on the web concerning the effects of max load vs real load and how things got better as you added safety margin to the load index. Longer life, better traction, and stopping as the slip angles decreased. The improvenments were roughly half of the percentage of load increase [1521->1640=7.8%/2 or 4% but the tread life increased by DOUBLE or 15-20%!..............3 or 4,000 miles is significant.

Actual testing [using them on many Q] of Michelin Pilot H4 in 215,225,235 x 15" on the Q shows this to be true.......even though the tires are constructed the same - a 98 vs 95 load rating is the superior tire in every aspect and the $20 difference pays for itself quickly.

An Eibach lowered Q is a great test bed as the -1.5 negative front camber eats up every tire. I test my tires before removing the old balance weights and keep a log of each tire and use the severity of out of balance as an indicator of internal changes. Not necesarily the poistion but the amount of weight changes as the tires slip/spin on the rims due to braking on the front and WOT acceleration on the rear. Every time I change tires I test the wheels for balance and run out.......... I started out with 2.5 sets of oem wheels [10 wheels ] and am down to 5 inspec wheels after 253,000 miles. The others were sold to people whose wheels were really bad not just 10-15 grams out of spec.

Over the years I have used Michelin more than any other brand: XGTV, V4, H4,H4,H4.......then YOK AVS I, AVS I, AVS I, & U+4 and a set of Firestone SZ50 non EP all on 15" wheels.......even tried Dunlop 5000 but gave them away in 5,000 miles.Many times I would have the AVS I on the front and H4 on the rear and every tire got rotated at least every 3,000 miles some every 1,500.

[Q45tech]

http://roads.dicea.unifi.it/co...3.pdfh ... co...0.pdf

[maxnix]

So, on reading the sidewalls on my Yokohamas and Firestones, it says "Maximum load carrying capacity X lbs. at (Maximum inflation pressure) p.s.i." (Omitting metric equivalents.)

So from where does this standard of 35 psi achieves maximum load carrying capacity come? The sidewall information seems to explicitly contradict it.

As a note, purchased Firestone SZ50 EP 245/50-16 from Tirerack for wife's 2000 Q45 and had Phil Smith expertly mount, balance, and align the car at his Les Schwab location. Perfect hook-up. She couldn't believe how bad her tossers were.__________________Brian1995 Q45 & Q45t & 2000 Q45

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[Grant@tirerack]

I'm going to cut and paste this from the link you found on our website and posted in an earlier response:

A tire's "maximum inflation pressure" may be different that the assigned tire pressure used to rate the tire's "maximum load". For example, while a P-metric sized standard load tire's maximum load is rated at 35 psi, many P-metric sized standard load performance and touring tires are designed to contain up to 44 psi (and are branded on their sidewalls accordingly). This additional range of inflation pressure (in this case, between 36 and 44 psi) has been provided to accommodate any unique handling, high speed and/or rolling resistance requirements determined by the tire and vehicle manufacturers. These unique tire pressures will be identified on the vehicle placard in the vehicle's owner's manual.

[Q45tech]

But the placard is for the oem tire, I would be very concerned about any substitute tire brand or model...........why I always recommend something stronger [load index] speed rating when substituting tires.

The other problem is identical branded tires in the aftermarket may not be the same as tires supplied to oem for use on a new car...........they may have the same name, model, etc but be constructed differently inside.............gotta be careful.

The Bridgestone RE92 comes in 6 different versions [speed rating and reinforcements/nylon caps] in the same size.

[Grant@tirerack]

That's an excellent example. O.E. tires like the RE92 or the Continental Conti Touring Ch 95 Ecology Plus have as many as 8 versions in the same size. They produce these to the car makers' specs and can differ in load rating, tread design, compound, sidewall and tread construction, and even physical size depending on the car they are designed for. Always make sure you are matching the load rating when you change size or are using the same size in a new tire model for the best results.

[maxnix]

I'm going to cut and paste this from the link you found on our website and posted in an earlier response:

A tire's "maximum inflation pressure" may be different that the assigned tire pressure used to rate the tire's "maximum load".

Perhaps, but by whose authority? And how does a consumer determine what the "assigned tire pressure used to rate maximum load" is? And, finally, why the explicit language on the sidewall that maximum load carrying capacity is reached at the designed maximum tire pressure (consistent with Boyle's law) and not at the arbitrary tire pressure used? After all, the sidewall is only rubber and the information can be easily changed for each mold.

Does any tire manufacturer directly address this issue other than with sidewall information?__________________Brian1995 Q45 & Q45t & 2000 Q45

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[szh]

Hi, Brian.

I may be wrong, but it is my understanding that the "Tire and Rim Association" (to which all the tire and wheel manufacturers belong) sets the guidelines - based on Federal requirements - about how the measurements are made and reported, etc. The Feds do mandate the information (like the character size, etc.) presented on the sidewall, though.

Z