Grant ... question for you in re my Infiniti Q

[szh]

Hi, Grant.

Some questions for you in regard to wheel and tire fitment. I have a 1995 Infiniti Q45 (stock suspension, no lowering, i.e., no special height or ride changes).

Currently, I have the stock Infiniti 15x6.5" rims with 45mm offset, on the car. The stock tire size is 215/65-15, but I am using Firestone SZ50EP in a 225/60-15 size that I got from Tire Rack, with excellent results. Reasonably close diameter, etc., and wonderful wet weather imporovement over my previous tires.

Some months back, I bought Infiniti 17x7 rims with a 45mm offset (off a G35 sedan) with the intention of using either 245/45-17 or 255/50-17 tires (and live with the nominal speedo error in that second size).

My question is: given your experience at Tire Rack, do you think that the either of these 17" tire sizes (given my new 17x7", 45mm offset wheels) is likely to be an issue with the 1995 Q45? Are these likely to rub on the suspension parts, or give me problems, etc.?

Z

[Q45tech]

Don'i think that because a tire says 255/50/17 that all brands are exactly the same width at each point on the curve.We see quite a few 90-96Q with 255/50/16 on up to 16x 8.5" wheels.17X7" rims are a little narrow for a 255/50/17 which needs at least an 8" wide rim.

!!!!!!!!!!!!!!!!You must find a tire with the oem load index [95V or higher and 1521 pound load rating at minimum].

The 245/45/17 SZ50EP meets the specs but few others do

[Grant@tirerack]

A 17X7 would normally be a 40mm offset. We don't do cross fitments on O.E. wheels since we don't sell them. We have never test fit the G35 wheels due to the offset so you want to check brake clearance behind the wheel. It may work, it may not. We've never attempted it here. If the wheel works I would stick with the 245/45-17 Sz50 EP. The stock tires have a load rating of 1378 @ 29psi. The SZ50 Ep would be fine at 1437 @32 psi. Q45Tech is exactly correct on the tire size. That size number is not an exact size. It is a 'range' and tires within the same size can vary by quite a few 1/10's of an inch in any direction. The load rating needs to be at least a 97V on the tires. The tire size and model would be correct, it's just the wheel on that car that is in question. 5mm is not a big change but there can be any number of issues with the wheel fitting. It may very well work but I can't give you a 100% without actually putting the thing on the car with a tire.

[szh]

Thanks, Grant and Dennis!

I appreciate your comments! I'd like to ask some additional questions though, since I think I have taken some of your thoughts into account:

Dennis: Must find tires with load index of 95V or higher.

Absolutely agreed! All the tires I am considering are all 95V or higher load ratings. In a 245/45-17 size, the SZ50EP is my current first choice, but I am also looking at a few others - all are 95V or 95W. Like the Yokohama AVS db S2, Yokohama ES100, BFGoodrich g-Force T/A KDWS and Bridgestone RE750. Although they do have differing Max Inflation Pressures (MIP) - see my question below.

Dennis: 255/50-17 needs at least an 8" wide rim.

I thought so too ... initially. But, at the Tire Rack web site, almost all the 245/45-17 tires show a width range of 7.5"-9", whereas the 255/50-17 specify a range of 7"-9" (i.e., a smaller lower width - see the info on Bridgestone RE750 as an example.) I attributed this to the taller aspect ratio that allowed a bit more curvature in the tire sidewall. Would you agree with this, or am I pushing it too far? The Bridgestone RE750 255/50-17 is a 101W rated tire, by the way, at a MIP of 51psi.

Grant: Check the brake clearance behind the wheel.

Yes, indeed! Before I decide to use these 17x7 rims, I was planning to put them on the car (without a tire) and see if it had any problems with the brakes and stuff. Since they do seem to have a lot more "room" inside the wheel compared to my existing 15x6.5, I am comfortable that this should be okay, but good to check, agreed!

Dennis and Grant: Tire width points on the curve are not the same for all similar size tires.

Good point. I was also looking at the stated section width's on the Tire Rack web site to see if I could figure it out. Unfortunately, there is not enough data to provide a clear answer.

Grant, on a related topic, I have always been confused by the Max Inflation Pressure description - the explanation at the Tire Rack web site and responses here have not been clear to me. It became a bit more confusing with your info that the SZ50EP "would be fine at 1437 @32psi".

So, what is the difference between a tire rated at 95W with Max Inflation Pressure of 44psi (like the Yokohama AVS db S2) and a tire rated at 95W with a MIP of 51psi (like the Yokohama ES100) or a tire rated at 95W with a MIP of 35psi (like the Continental ContiExtreme Contact) ... all at 245/45-17? If the tires are all used at the same actual pressure on the car, does this mean that the Continental has the highest load capacity at ... say ... 32psi?

In other words, what does "Max Inflation Pressure" really mean? How does it relate to the load rating?

Z

[sultan]

i've got 17x7.5 +40 offset with BFGoodrich g-force KDWS 255/50-17 tires on the back. no rubs, even when i still had the eibachs. i haven't tried them on the front, however.

[szh]

i've got 17x7.5 +40 offset with BFGoodrich g-force KDWS 255/50-17 tires on the back. no rubs, even when i still had the eibachs. i haven't tried them on the front, however.

What do you have on the front?

Z

[Grant@tirerack]

The maximum pressure listed on the tire is specific to that tire model, not the vehicle. That's the maximum pressure the tire could handle when tested under load without failure. The correct pressure to run is going to be based on the vehicle. The vehicle placard or owners manual will list the correct tire pressures to run front and back based on the in-house testing they do on the vehicle. This is based on the weight distribution front-back and loads on the tires in motion. The maximum pressure tires can handle will vary depending on how the tire is constructed. Even in the same size, tires can sometimes handle more psi if there is a heavier sidewall and belt construction. That's what that load rating number is on the sidewall. The tire's ability to handle load is determined by a combination of air chamber size and the air pressure you use to fill the chamber. A 97 load range tire in the 245/45-17 size can only handle 1351lbs per tire at your car's stock air pressure. This is because the 245/45-17 has a slightly smaller air chamber. Since it has a higher maximum air pressure rating, you can bump the pressure up to 32psi and it will increase that load capacity up to 1437lbs which would be fine. You would need to select a tire that can handle up to 35psi on the max. pressure and has that 97 load rating. Some of the tires in that size do not have a high enough rating. An 89 load rated tire can only handle up to 1279lbs at 35psi. When in doubt, call us or a local tire shop and we can figure out the load needed.

[szh]

I think I have it understood now ... maybe!

If my new understanding is correct, what you are saying ... to simplify (without taking the car into account) ... a given tire rated at 1521lbs at 44psi Max Infl Pres has a load capacity of 1437lbs at 32psi and 1351 at 29psi. Is this a correct calculation?

Z

[Grant@tirerack]

That is correct. Many times you can safely run a smaller tire on the same vehicle if as long as you can up the air pressure and the tire is rated for the same load.

[szh]

That is correct.

Ah, good! I am on the right track ... I think!

1. So, how does one calculate this derating of the load capacity? The values you used are certainly not a linear fit, right?

2. If a tire is rated at 1521lb with MIP of 51psi, does this mean that its load capacity at 32psi is less than the 1521@44's capacity of 1437 at the same 32psi?

BTW, thanks for putting up with my novice questions!!!

Z

[Grant@tirerack]

Actually I have a cheat sheat to calculate the load rating. There is an industry standard tire guide that breaks all the sizes down by size, psi, and load rating. I'll check to see if that is online somewhere and makeit a FAQ link.. Looking at the same 245/45-17 size:load index 95 tires can handle the following loads at the psi listed1261@26 1351@29 1437@32 1521@35

load index 89 tires can only handle the following loads1113@26 1179@29 1235@32 1279@35

As long as the load matches the stock tire load rating and you are not exceeding the tire's max. psi rating, you are fine.

[szh]

Ahha! These numbers look linear now - i.e., capacity increases as pressure increases in a linear way! Okay, I think I am starting to see it even better.

But one question now begs to be re-asked in a different way: Two tires ... both load rated at 95 ... one has MIP of 44psi and the other is 51psi ... they both have the same load capacity at 35 psi, correct?

If so, then the Max Infl Pressure is just a maximum number for inflation pressure that is apparently not related to the load rating!

This is turning out to be an enlightening thread for me. Thanks again!

Z

P.S. I would love to see that cheat sheet that you mention! Details on how load capacity is calculated would be wonderful. Thanks!!!

[Grant@tirerack]

Oh my head is starting to hurt:D If the tire has a higher max. psi rating it would, in theory, have more load as the pressure goes up. Never exceed the max pressure on the sidewall and you're cool. Most street tires will show 44psi on the newer models. That's quickly turning out to be the standard. By the way. The SZ50 EP in a 245/45-17 is going to need at least a 7.5" wide rim to seat the bead safely.

[szh]

Oh my head is starting to hurt:D

Sorry!

But I am learning a lot - thanks for the teachings! Quote »If the tire has a higher max. psi rating it would, in theory, have more load as the pressure goes up. Never exceed the max pressure on the sidewall and you're cool. Most street tires will show 44psi on the newer models. That's quickly turning out to be the standard.[/quote]Yeah ... but ... a lot of tires at your site show a 51psi max now - this did not used to be true not too long ago!Quote »By the way. The SZ50 EP in a 245/45-17 is going to need at least a 7.5" wide rim to seat the bead safely. [/quote]Argh! Some of the 255/50-17's (like the Bridgestone RE750) show that they will work with 7" minimum width rims. Should I go for those instead?

Z

[szh]

Hi, Grant.

I'd love to see that cheat sheet you mentioned. Any chance you can post that? Or e-mail me a a copy to understand and learn?

Z

[Grant@tirerack]

It's actually about 10+ pages long and I'm still trying to locate it on the web somewhere so I can post a link to it in the FAQ section. I would stick with the models listed for a 7" wide rim.