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Old 08-13-2008, 09:26 AM   #1 (permalink)
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How much does tire and wheel weight effect MPG?

If I remember correctly, putting light weight tires and wheels on a car will increase your 1/4 & 1/8th mile times on the track as well as slightly reducing your stopping distance.
The new 2009 Escape Hybrid Limited has 20.4 pound wheels and 29.4 pound tires. There are 18.1 & 16.4 pound lightweight SUV wheels available from TireRack. Is there any way to figure out how much putting lighter wheels on would benefit fuel economy?

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Old 08-13-2008, 09:39 AM   #2 (permalink)
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Hi,

Wheel and tire weight count for 2X weight elsewhere in the vehicle. This is because you have to move the wheels in two ways: they are part of the overall vehicle weight AND you have to spin them.
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Old 08-13-2008, 10:37 AM   #3 (permalink)
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Tire/wheel weight is most crucial during acceleration and deceleration. If you do a lot of stop and go you are much more likely to see FE increase. Once the car is up to speed the mass of the tire and wheel will tend to stay in motion so the weight saving is of little consequence. Aerodynamics will almost always win out over weight in real world applications. If you are looking for a huge increase, it probably won't do it for you. If you are looking to do everything possible to increase FE then it should definitely be considered.
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Old 08-13-2008, 10:43 AM   #4 (permalink)
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The answer is there's no easy formula to figure it out. Also, the effect of just going to lighter wheels is probably fairly small (not saying it's not worth doing).

Weight reduction will have the biggest impact in city driving, obviously. As an example: GM just released "XFE" versions of two of its trucks, and lighter alloy wheels were part of the new mods package. They reduced mass elsewhere and added LRR tires too:

Quote:
Aluminum lower control arms (reduce mass)
Aluminum spare wheel (reduces mass)
Seventeen-inch aluminum wheels (reduce mass)
Automatic locking rear differential
Low rolling resistance tires
Result: 7% improvement (+1 mpg from 14 mpg) in the EPA city rating.

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Old 08-13-2008, 11:54 AM   #5 (permalink)
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wheels / mpg

Somewhere,I've got an SAE paper which deals with what you've asked about.Polar-moment-of-inertia is the physics part of your investigation.A special,3-wire pendulum is used to ascertain the value,and it has never been published info.As you've heard,the tire/wheel combo is essentially a flywheel,and the more mass,the more power required to accelerate,and decelerate.The SAE paper allows for an "equivalent-mass" to be estimated for the combos,as a function of vehicle mass percent.For an "intermediate-sized" car (used to be in the 3,500-pound inertia class ),a 10% weight reduction would net you a 1-mpg improvement.Thats alot of weight,and changing wheels and tires would never get you there.In town would yield the highest benefit,on the open road,it's basically a wash.Adding 1,200-lbs to my CRX only cost me 2-mpg on the highway.I'll try to dig out the article.
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Old 08-13-2008, 06:57 PM   #6 (permalink)
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The car I mainly use to drag race is a lowly 84 Horizon...my son gave me a set of great looking Konig 17s with 205/40/17s on them....looked great, but my car slowed down nearly a half second on the first two races out....that set of 17s were 46 lbs per corner and the ugly 13s I had on there of the same width and diameter were only 26 lbs per corner! A half second in the quarter mile is HUGE!
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Old 08-14-2008, 08:39 AM   #7 (permalink)
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aerohead is right, polar moment of inertial is the acceleration/deceleration "weight" that heavier tires & wheels saddle you with.

The trick with polar moment is that it isn't only weight, but where the weight is relative to the rotational axis that is important. Say you had a 40 lb tire/wheel combo that was shaped like this: <> only taller/skinnier like a soap box derby car, it would have a much lower polar moment of inertia than a 40 lb tire/wheel combo shaped like an I.

The trick is to keep the mass as close to the rotational axis as possible. This is the reason that going to larger diameter wheels can drastically affect your mileage and acceleration time even if the tire OD stays the same, the tire weight is pretty constant but with a lower aspect ratio that mass is concentrated further from the rotational axis thereby increasing your polar moment of inertia.
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Old 08-16-2008, 03:01 AM   #8 (permalink)
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all engines with counter balancers with the 50% "play with itself" design fail with heavier wheels and tires. A 3 main boxer is the only one I have ever encountered that benefits because the engine is true zero'd at all times, all rpms, nothing lost, all gained, and then there is the mythical momentum that extincted it so honda could make a buck with its wiggly little hubcapped crap and folks brainwashed. Uhm. Yeah go for lighter wheels, bad engines need it.For every little wheel wiggling its way in the fast lane today by me bragging about gas mileage I think of all that is fought to death for my country ...how seriously insane am I headed?
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Old 08-18-2008, 03:48 PM   #9 (permalink)
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wheel/tire/weight/mpg data found

I located the materials I was thinking about,set them aside this morning,then left for town without them.Typical for Phil! I will bring them next time and post in sticky at top of page.

----------------------------------- Here's the crux of what they report: The "equivalent mass",do to the rotational inertia of the four wheel/tire combinations,constitutes an addition of 3% of the vehicle's weight.

----------------------------------- For the 15 vehicles included in the study,the average aggregate vehicle weight was 4,300-pounds,and each wheel/tire combo added the equivalent of 15kg (33-pounds),or, 60Kg ( 132-pounds) to the total vehicle weight.

---------------------------------- If one could buy a set of wheels manufactured from Unobtainium alloy,and tires constructed from Extraordinariumbutidine synthetic rubber,having no mass whatsoever,the savings to the motorist would be: 1.3 % mpg in urban driving,and 0.7 % savings at a constant 70-mph ( 111-km/h ).

---------------------------- Since we must settle for cast,billet,or forged Aluminum wheels,and conventional synthetic rubber tires,it looks as if weight savings in this arena will provide only dubious benefits.

------------------------------------ Lower rolling resistance is a worthy goal if it can be purchased,however mass doesn't appear to play any real significant role in potential for mpg improvement.
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Old 08-18-2008, 07:13 PM   #10 (permalink)
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I wondering the same thing about wheel weight as I switch from 14in rims in the winter 20in rims in the summer.

What I have found is that I actually get better mileage with the larger rims, but only when driving mainly highway speeds and P&Ging. Because I can coast farther and take turns at faster speeds without using the brakes I think it also helps the mileage because I can put 55psi in the tires as apposed to 35-40psi with the 14 inch rims.

My question is does anyone think that the inertia of the heavier wheels is the main reason I am seeing an improvement in mpg or is it the higher psi in the tires ?

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