How much does tire and wheel weight effect MPG?

[twolostminds]

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?

Mike

[NeilBlanchard]

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.

[bikin' Ed]

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.

[MetroMPG]

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:

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.

Source

[aerohead]

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.

[ATaylorRacing]

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!

[MechEngVT]

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.

[bgd73]

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?

[aerohead]

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.

[BetsyBio]

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 ?

[whokilledthejams]

Originally Posted by BetsyBio

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 ?

It's probably a combination of the two, as well as the driving you do. If you did a lot of stop-and-go, you'd see better results from smaller, lighter wheels. Really big alloys are extremely heavy, though ricers will swear up and down that the aluminum 18s they put on their beater Accords make them faster, even though they have twice the weight to turn vs. stock steelies.

Also, winter gasoline formulations will tend to make your FE take a hit, too. You might try running a couple of tanks in the summer with your winter wheels as a control.

If you can properly take advantage of the inertia offered by huge wheels, however, then go for it.

[Fast Guy]

Originally Posted by BetsyBio

I wondering the same thing about wheel weight as I switch from 14in rims in the winter 20in rims in the summer.............

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 ?

I think it's probably due to change in gearing with the 20" wheels. I'd imagine they'll be a fair bit bigger than the 14"s in diammeter, so your engine should be doing lower revs for the same road speed. The higher psi may contribute slightly due to the reduced friction.


(sorry for the late reply)

[Fast Guy]

Originally Posted by twolostminds

If I remember correctly, putting light weight tires and wheels on a car will increase your 1/4 & 1/8th mile times

Mike

I think you mean reduce those times. My lighter wheels reduced my times by about 0.15secs



(ooops, think I hit quote instead of edit)

[almightybmw]

Originally Posted by Fast Guy

I think it's probably due to change in gearing with the 20" wheels. I'd imagine they'll be a fair bit bigger than the 14"s in diammeter, so your engine should be doing lower revs for the same road speed. The higher psi may contribute slightly due to the reduced friction.


(sorry for the late reply)

why do people think bigger rims = bigger tires? this isn't a sliding scale where 185/60R14 is equal to 265/60R20. The goal is same tire/rim overall dimensions. The 14" overall is 23 inches. To match that on a 20" rim you'd need a seriously small sidewall, like 15. hmm, I could run a 195/20R20. 205/15R20 puts me a hair slower...ah! 235/15R20 is the perfect fit. Now, if only I could make a 235 fit in the wheelwell where a 205 rubs....

thank you for the entertainment, I'm now going to try and find a 20" rim that's 6.5" wide instead of 9-10". should be a challenge worthy of my time.

[Fast Guy]

I know bigger rims don't always mean bigger overall diammeters. On average most people go down 1" or 2" and you go down around 1 profile per inch to maintain a similar overall diammeter. Going up 6" on the wheel alone will be a problem if you're starting with low profile tyres already.

You must have different tyres to what we can get over here as I've never heard of a profile lower than 30 or 25. As far as I knew 15 and 20 profiles didn't exist. So any 20" tyres are going to be bigger than the 14s. If you have incredibly small profile tyres over there, then fair enough. Give me a link as I can't find any.

Maybe BetsyBio can tell us what tyres she has on the 20" wheels to clear up any confusion.

A 185/60R14 works out at 577mm diammeter
A 225/30/20 (smallest I can find) is 643mm and that is going to affect the gearing.

I fitted 215/35/16s to my 16" wheels, they are 556mm diammeter, smaller than quite alot of 14" tyres and 30mm smaller than the tyres that came on my standard 15" wheels and they have helped my acceleration due to being a smaller diammeter.

[dichotomous]

I went from 32mpg in my civic Si with 195/60r15 winter wheels at 30psi to 29mpg with 215/60r15 tires at 32psi (though going from 30 all the way to 42psi didnt change it much) .

I drive mostly cruising around 50mph but there is about 1/4 to 1/2 city driving, I'd say its a pretty good combined.

I've got a different set of winter tires, same style but 205/60r15 and have them up to 45psi, so we shall see on that, though, its an unfair test because I am also running now a raceheader and I've changed my rear wing to act as an extended decklid instead of a downforce/drag creator

[CobraBall]

Originally Posted by Fast Guy

You must have different tyres to what we can get over here as I've never heard of a profile lower than 30 or 25. As far as I knew 15 and 20 profiles didn't exist.

Kumho 385/15-22

They exist but how practical, it don't know. Probably ride like solid metal.

[Fast Guy]

Originally Posted by CobraBall

Kumho 385/15-22

They exist but how practical, it don't know. Probably ride like solid metal.

Well that's a start even though it's a 22" tyre, but I doubt there's anything much narrower like the 195/20s, 205/15s or 235/15/20s made as the almightybmw implied with his sneering attitude, as the sidewalls would be so small that a) they'd be a real pain to fit to the wheels (my tyre fitters struggle with my 215/35s) and b) any hard cornering and the wheel could probably rub on the road due to tyrewall flex.

I could run a 195/20R20. 205/15R20 puts me a hair slower...ah! 235/15R20 is the perfect fit. Now, if only I could make a 235 fit in the wheelwell where a 205 rubs....

thank you for the entertainment, I'm now going to try and find a 20" rim that's 6.5" wide

If you want 6.5x20rims, custom build will be the most likely option.

mind you, when I first started modding cars, the only place you could find 20" wheels was on a lorry. (semi/big rig or whatever they are called in the USA)

[Bearleener]

Tire & wheel weight will definitely have a slight effect and, as some have pointed out, each pound on the wheels counts more than, say, a pound in the trunk (or in the driver's seat). And the more accelerating and braking you do the more it matters. Don't forget also that increasing the unsprung mass leads to additional losses because the tires must absorb stronger hits when going over bumps. Also, if the tires & wheels are heavier because they're fatter, then the aerodynamics will suffer, too. And cheapo steel wheels with stock hubcaps probably have better aerodynamics than most cool-looking alloys.

A little theory:
The linear inertial energy of a car (or its wheels&tires) is
Elinear = (1/2)*m*v^2 ,
where m is the mass and v is the vehicle speed. That means you have to feed in at least that much energy (from the engine) to accelerate the car up to that speed.
Additionally, the rotational energy of a wheel/tire is
E rotational = (1/2)*I*omega^2 ,
where I is the rotational moment of inertia (see List of moments of inertia - Wikipedia, the free encyclopedia) and depends on the mass distribution.
In the worst case all the mass is concentrated at the outer radius (a hoop); in this case I = m*r^2 where r is the radius.
An intermediate case is uniform distribution (a disk, probably a bit optimistic); I = (1/2)*m*r^2.
The ideal best case would be a point mass concentrated at the axle (I = 0), but you can't build a wheel like that (except out of unobtainium).
The rotational speed of the wheel in our case is omega = v/r, so for the hoop we have Erotational = (1/2)*m*v^2 and for the disk it's Erotational = (1/4)*m*v^2.

So comparing to the linear energy you can see that each pound in the wheels & tires indeed counts 1.5 to 2 times.

Some practice: Every year when I switch from summer to winter tires (both 185/60R14 from Continental, same circumference) I notice that the car accelerates a bit differently in 3rd gear. With the summer setup (alloy wheels, weigh 1 kg (2.2 lb) less each; i.e. 15 kg instead of 16 kg for wheel&tire) the car has a surge of acceleration in a certain range of engine speed, whereas with the winter wheels&tires (steel rims) the acceleration is more constant. So even the 6 to 8 kg of equivalent additional mass can be felt on the butt dyno. The perceived difference is more pronounced even than adding a passenger. Go figure!

What I also immediately notice is that my winter tires actually have *less* rolling resistance than the summer ones, even though I'm running the winters at lower pressure. I suspect it's because they're a newer model, where they paid more attention to fuel economy when designing them.
BTW on an earlier thread others found the opposite Summer vs Winter tires , but remember that in the winter other factors (longer warm-up time, less-potent winter gasoline, more air drag at a given speed in colder air) will also take their toll on fuel economy.

[Frank Lee]

Originally Posted by bgd73

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?

wth?