A 26" unloaded tire on what ever wheel will act like a 25.2" x pi = 79.168" when the vehicles weight compresses the tire.

5280 x 12"= 63,360 inches per mile / 79.168" = 800.32 revs per mile

800 tire revs per mile x rear diff gear ratio [say 3.538] = 2831.544 drive shaft revs per mile then correct for highest tranny gear ratio say 0.694 yields 2831 times 0.694= 1964.7 rpm = torque convertor slip =~2000 rpm for engine.

The point is you look at the rolling radius or the revs per mile.
A tire/wheel combo that creates 5% less than oem revs per mile will reduce engine rpm by 5% for same speed and this will reduce fuel consumption by <5% .

Unfortunately there may not be much extra clearance for a larger tire/wheel combo, but there is usually room for some increment on the rear since they are not steered like the front.

3.8% say 27" vs 26" isn't very significant in the MPG scheme of things.

Heavier wheels/tires just slow down acceleration and increase stopping distances and friction between tire and road so much of the MPG gains are lost to other factors.

Luckily illeducated are allowed to remain alive in most countries.

Someone posted "65% of engineering freshmen change majors after seeing workload.

US grads 468 per million pops Bs Engineers per year or ~~ 140,000 per year

Breakdown of BS degrees by specific concentation very interesting;

http://memp.pratt.duke.edu/dow...x.pdf

The only way you can know for sure is to conduct a series of coast down speed distance tests with various wheels/tire combos.

Start at 60 mph put car in neutral and measure the distance to a full stop on a perfectly level airport runway.

Dry, wet, in snow and ice.

It would be more beneficial to find a very lightweight rim/tire combo with short narrow rims.

Even if you were able to lower your RPMs in your final gear, how much fuel will you have to inject in order to maintain your velocity in order to overcome wind resistance and other "frictions". You may find that your engine runs 1k rpms less, but it's also injecting more fuel into every stroke to compensate for the lack of torque.

Somebody please correct me if I'm wrong.

F/S:G35/350Z/FX35/Altima Grounding Kits

What are you running? Brake Setup Poll -- Plenum/Spacer/Intake Manifold

As far as the larger tires increasing mileage I think that's been answered but I say no. It's about rolling resistance not diameter. What will increase your MPG is getting a harder narrower tire which is the opposite from the tire most of us will choose for our performance cars. Now for my little Toyota p/u daily driver, I have narrow, hard compound tires and I over inflate a little. Along with some driving techniques and other tricks I get up to 32 MPG compared to the EPA standard of around 22-24 mpg for this truck.

Contact patch area is JUST a function of weight on the tire and PSI............wider just changes the shape of contact patch not the AREA.

900 pounds on a 35 psi tire has a 900/35= 25.714285 square inch contact area [patch].................why higher inflation reduces the area.

I might be missing something crucial here, but I find that hard to believe.

Yep. It's the worlds greatest misunderstanding.

What Q45Tech is talking about is balloon theory. However there is plenty of empirical data that has been done that shows otherwise that has come from motorsports. It has alot to do with the tires construction and overall parameters surrounding the car such as it's wheel size. However for the most part many people adhere to the balloon theory.

It would say that a 185mm tire with 35 PSI should have the same contact patch as a 355 tire with 35 PSI. As such, there is not reason to go for wider tires in order to improve traction.

As long as the tread didn't wrinkle, thereby lifting a portion of the adjacent tread, then the friction between the two widths should be very close.

The gain in traction of the tire is involved with dynamics. There is a point where additional pressure on the tire does not yield a 1:1 response in grip. This has alot to do with the tires load index.

The point to be made is that just because a tire is wider does not necessarily mean it will produce more grip. There is a limit to the return depending on the maximum potential of the car. The main reason why F1 cars and other race vehicles use incredibly wide tires is due to the presence of downforce. Street vehicles don't have downforce, and as such they are easily "over-tired".

A 2200 lbs. track car will not require the same load capacity as a 3200 lb. track car. It has alot also to do with track width, wheelbase, center of gravity, and other factors that effect dynamic weight shift. A 2200 lb. Miata for example will exhibit more weight shift due to it's small wheel base, and thus can make greater use of wide tires than a 2200 lb. 240SX.

The secondary reasoning behind wide tires is increasing the surface area of the tire to reduce thermal stress. This is purely a track issue.

My experience with wider tires yielded me better traction and less hydro-planing. I'm having a hard time digesting this information.

The reduction in hydro-planing is because of the tread, not the wider tire. A wider tire is worse in the wet / snow. This is because the patch is wider, giving less time for the tread to evacuate water.

Sometimes people that move up to a fresher, better, or much taller tire associate it with additional grip. It's like replacing an incredibly dirty OEM airfilter with a K&N and experiencing power "gains". Bear that in mind as well.

Manufacturers are putting wider tires on the cars because tire technology as well as suspension design are increasing the ability of the cars to make use of the tires. In the case of sports cars some of it may be lent to marketing appeal as people associate wide tires with ability. When you look at the average gain in vehicle size and weight, it is no mistake that tires are also increasing in size to maintain load capacity relative to the vehicle.

Force = (Pressure)*(Area)

The force (weight) your car exerts does not change and the pressure is the same (35psi) so the contact "area" not patch has to be the same.

If you were to make a stamp of a street tire patch, you would have to estimate the area of every individual tread blocks in the contact patch and add them all up to get your contact area. This only pertains to a parked car

Tire dynamics alone with out considering suspension tunnig are very complex. There are so many forces and torques one tire experiences when driving. The complexity is in analyzing a deformable body like a tire. So like nismo freak said wider is not necessarily better.

lateral forces in the tire come from a distortion of the print due to slip angles. As the tire is turned against its heading the tread coming into the print is streatched laterally...

You mat turn the tire but the road keeps wanting to pull the rubber in the print backwards (with referance to the trajectory of the tire, not the actual plane of the wheel) as it deflects its almost like a spring, and pushes back against the ground, this is what is changing the direction of the car.

As the width of the print increases so does its resistance to this lateral deformation, this means higher lateral forces at lower slip angles.

Kinda a ****ty discription...but I am tired, and enegy drinks make my brain feel scrabled when I have to think

I'll try and get a picture diagram up to make it more clear. Books back at the apartment, and internet in my room isn't working

Edit: Like this


Modified by Red coupe at 3:45 PM 6/25/2008

The complexity of the tire isn't well understood. There was alot of F1 tire testing done back in the 80s and 90s in regards to aerodynamic load and how the tire sizing developed it's response. There was alot of misconception they found in regards to how stuff reacted with heavy loads. In fact, it pretty much illustrated that there are no guidelines to tire grip due to the heavy reliance upon construction.

To be completely honest only semi learned, I have read a good chunk of that book...but that was back when I could only look at the math and go "ahh yes... that S looking thingy..." and I still wouldn't be clear on all the suspension parts he was talking about (i.e. name have to look up components he named some times...)

Now that I am done with my math and physics, and am progressing though my core ME classes I have started going back through it since I can acuatlly understand more then just the basics of what is being said/disect the forumulas.

Its a page turner though

Part of the industry does, but there are many companies that produce quality components as well.

Some engineers lack the ability to think outside of the box. They are stuck on stupid with regards to OEM.

When comparing two diffrent width tires with the same construction, materials, side wall profile and diameter.
EX 200mm VS 315mm

Using ballon theory if I fill the 200mm tire to manufacters "spec" tire pressure (ex 35PSI) and then I fill the 315mm to 35psi I will get the same contact patch...

But if I fill the 200mm tire with only enough air to create the best potential for traction, 25PSI
(or 1 Pound per Square MilliMeter?...)

200mm(width) x 75mm(Profile) x 1(PSmm) = 15,000mm of air or the amount of air in the tire.

Now if I filled the 315 width tire with 15,000mm of air, would I then reach the "best potential for traction" on the 315mm tire?

Would my theroy hold tru?
I came up with this theroy after reading the thread. I am sure there are still many variable that would affect my results, but did I get the jist of it?

Quote, originally posted by Q45tech »

The aftermarket industry DEPENDS on the typical buyer not understanding even high school physics.

Is that a manditory class in HS?

I never took physics.
Only geo, trig and pre-cal

Balloon theory is a gross generalization. It's the exact same as power vs. weight in that it does not address the details within the actual empirical usage of the item.

The problem is that if you have similarity in construction and materials then you will simply narrow it down to some degree in error when utilizing the balloon theory. I say this because with changes in width and sidewall you are altering the structural ability and response of the tire, and that is where the balloon theory does not apply. F1 engineers noticed this when they did actual load testing on the tires. Granted, F1 cars undergo SIGNIFICANTLY more load than any street vehicle (until you hit potholes and curbs).

For what it's worth, here's a tire load vs. slip angle test. See the linear and un-linear relationships.

Thanks allot! I really do appreciate your response.

If you want to learn the easy way, get a book called Race Car Vehicle Dynamics. It's $100, but it's value is worth 10-fold.

You can buy it on Amazon, most book stores will not have it unless they carry SAE textbooks.

Worth while book for sure, and even with out the math/physics background to understand all of it I would say that it is still worth while, but you may end up just staring at some of the pages like

I will say that that book is one of the reasons I changed my major to engineering though. I was sitting there when I realized... "wait, this is basically some ones course work... I want to study this!"

I am still on Chapter 1: Basic Principles of Physics and I through most of it.

:D

Didn't really know what I was looking for, just knew I liked how things worked (i.e. how economy works, how buisness worked, how peoples brains worked...and how race car works :D ) Those were all at the JC level though, and none but econ went past an intro class....even econ I only had a few classes in.

When I talked to the councilor towards the end of my JC period she was amazed how many units I had, and that I hadn't taken more physics... I spent a few years studying everything from music theory to engine rebuilding and emissions controls.

I like what I am doing now though

Hahaha. I know the feeling. If money weren't an issue, I'd pursue a Masters in Mathematics, a PhD in Psychiatry, and a BFA in Graphic/Multimedia design.

Lately I've been hitting up wikipedia for TOO much info. I wish it was more reliable, but for most of my subjects, it seems to fit the need.

Wider tires will reduce your gas mileage, because the added width adds friction between the tire & the road. Good for handling, bad for mileage.

Going up in rim size (like say from 15" to 17") will reduce MPG because the larger rims will be heavier, and will move the weight further out from the hub. In addition, lower-profile tires have to be built heftier to compensate for their shorter sidewall, which adds weight out near to the edge of the tire. Rotational weight (tires, rims, brakes rotors, etc.) count for approximately four times as much weight as vehicle weight. So adding 10 pounds on the wheels, is like adding 40 pounds in the car, and added weight reduces MPG.

This is not always true. For a couple of reasons. Some engines will work much more efficiently at a higher RPM with light load, than a lower RPM with heavy load.

By increasing the overal tire diameter, you have increased the moment of inertia which increases the amount of torque needed to move it. You've also put the engine at a point that is probably less powerful and therefore will need more fuel to produce the same speed.

The 4:1 ratio is also debatable. It is extremely important to note where the weight is on th rotational body. If the weight is very close to the point of rotation than it's "weighted value" is nearly the same as chassis weight. On the other hand, if the weight is farther out on the torque arm, then it's "weighted value" is much higher.

The difference between rotating mass and static mass differ greatly depending upon the acceleration. The lower the acceleration, the less difference exists between the two.

Let me ask you; do you get tired faster running on concrete or in sand?

Assuming you have same type of tire construction, same tire pressure, same rolling radius and same weight, the larger rim makes for a "stiffer" wheel and therefore it's more like concrete than sand. Larger sidewalls, heavier wheels, lower tire pressure, softer road substrate all make for more "work" your car does in the form of spinning, flexing and squishing things; mostly the tire and rim itself. This = lower mpg.

So does an increase in the moment of inertia. I think everyone is missing that there are two areas to look at when addressing this question:

City style driving- Lots of accelerating and decelerating (shouldn't be a word)

Highway style driving- Mostly maintaining a constant speed.

For the latter, the larger rims may actually benefit the MPG because of what you mentioned regarding work. For the former, larger rims will cause a higher moment of inertia because the rims require more mass as they get "taller".

This is where CVTs pwn hehehe. Sorry I've been reading too many s-chassis threads lol.

Ever spun a 25lb 18in metal disk from a stop to ~400rpm in a couple of seconds?

I'd have to do more math to give you an exact answer... but I can give you a fairly firm guarentee that the second is harder, if even doable then consider that X 4.

Rolling resistance is a lot less then it takes to accelerate a wheel.