Effect of weight of wheel+tire
 

Please confirm or correct me on this -
Weight of wheels + tires has an affects acceleration and affects mpg when accelerating, but has a nearly insignificant effect on steady-speed driving?

For steady-state driving, reducing the combined weight of all wheels by 20 lb. would be the same as removing 20 lb. of unneeded cargo from the car?

But for accelerating/decelerating reducing that weight would have a more significant result?

I ask because yesterday I weighed the 2 different wheel/tire combinations I have on hand. Nice 205/65-15's turned out to be 4.5 to 5 lb heavier (each!) than 195/75-14's, both mounted on oem steelies. The two have nearly identical outer diameters and so are swappable without affecting speedometer/odometer accuracy or effective gear ratios.

It sounds to me like you have the right idea there just by thinking through it logically. The only thing I see that would be an advantage for heavier wheels would be while EOCing because they will have more momentum and thusly continue to role longer without assistance...but I don't have any proof I'm just thinking it might act similarly to a flywheel on a car. Heavier flywheel equals better gas millage then a lighter one because it can idle at lower RPM without stalling.

Hi Bruce,

Yep, that is right, to a first order approximation. The issue with steady state driving is that its not really steady state due to road imperfections, merging and road hazard avoidance. So weight will effect the mileage there too, but as you say the difference will be nearly insignificant.

On a Prius the effect is about half as much, as about half the rotational (wheels) and linear (total mass) inertias can be reused. In a standard car however, all that inertia is turned into heat by the brakes.

There are very expensive forged alumimun and or magnesium alloy wheels that can give you almost the same weight with the bigger heavier tires as with the smaller tires on cast alumimum alloy wheels, let alone steel.

Shop around, there may be some lighter weight tires, too. Although, they tend to have the lower tread life.

They also won't be as good at holding up to the higher pressures many of us run to help reduce RR. So if you want to run over the factory recommended pressure for your car make sure they didn't skimp on the thickness of the sidewall too much to reduce the weight.

The 195's will give you lower aero and lower RR. Then the 205's.

185/80 would be even better for aero and RR.

For a given overall weight the greater the distance from the centre of the mass ; the higher will be the centre of inertia.
So the greater the amount of energy needed to get things moving.

Also a factor but I think a fairly small one.

Cheers , Pete.

Thanks all.

My winter tires which just came off are the 195's. Summer tires are on as of today, 205/65-15. Relatively new tires, spec'd for 44 psi that I ran at 50-53 last fall. Not a low profile tire by modern standards but they do handle a lot better than the skinnier taller-profile 195's. Same overall diameter.

This is a 3000 lb. car that came stock with 185/R 14's which is equivalent to 185/80-14. Size is not easily available at good prices. It's a very tall sidewall by modern standards. For improved handling many drivers of these Volvos have gone to much larger (and wider) wheels to get less flexing of the tires in hard cornering.

Nowadays any 3000 lb car will usually have tires size more like 205/55-16 or 215/60-16. That's as wide as my 205's or wider, and a shorter sidewall.

So uh, would a heavier wheel coast farther than a same-sized lighter wheel on the same slope with the same environmentals etc?

I coast a lot farther with a full Jeep, but I have to be heavier on the pedal when accelerating.

Too bad it's impossible to make a car that adds weight while coasting downhill and while slowing down and removes it while going down the highway and accelerating. :\

Yes.

The 205 is of course slightly wider, but it's only an extra centimeter. If you're mostly driving at cruising speeds, not a lot of stop-and-go, then there probably won't be much difference between the two.

extra centimeter (X4) for each tire adds a lot to the "frontal area". Plus the added weight. Also more contact with the road increasing rolling Resistance. Even though it seems small when you look at only one tire, the total out come is different. You will see a drop FE guaranteed.:(

I went from 195's to 185 LRR's. While that single centimeter on the nominal width is a only 5% reduction, the actual tread width of the new tires is 19% narrower. 5.25" vs 6.5"

I also got rid of the "big 15's" and put on 14" rims. I wish I had before & after weights.

wheels and tires
 

The big boys refer to polar-moment-of inertia,which has to do with the mass of the wheel/tire combination,and where the mass is with respect to the center of rotation.They place the unit in a three-wire rotating pendulum,twist the thing up,and then time the frequency of "swing",to quantify inertial effects. Acting like four flywheels,its true that during "transient" episodes,when the car is accelerating or de-accelerating,either the engine or brakes must overcome the effects of the momentum due to the inertia of the wheel/tire combo.And I agree with whoever says,that,once up to a constant cruise speed,the mass of the units has little consequence to mpg,all else being equal.Composite wheels demonstrate the lowest mass,even against magnesium,have already received D.O.T.approval in the U.S.,and appeared on the market in the 1980s on a Chrysler:Dodge Shelby something or other.I tried to locate some for my exhibit and struck out.They seem to have bombed in the market,I don't know why.There are alot of things I don't know about!

Yes like everyone said, the light weight wheels affect mpg more when there is more accel and decel. In city driving, there is a lot of this, so it would make a more sizable difference. The VX, which is designed with fe in mind, has light weight wheels and small tires, and since everything else on the car is basic and simple (except the engine), I figure the difference must be significant enough to warrant the expensive wheels. honda could have made the car more aero, but they didn't do much. who knows why. I think that the light wheels are a good match for the tall gearing. It makes accelerating from low speeds with tall gearing easier. I notice that the car is really easy to stop at low speeds...this is partly just because of the light weight of the car, but probably also the light wheels.

I've heard that for every pound of unsprung weight you can remove (wheels, tires, etc) its like removing ~3 lbs of weight from the vehicle. But you know what people say.

It makes some sense though. Yes, you can coast further with a steel wheel since you have a higher polar moment of inertia. But you paid for that when you fought it while putting in power to accelerate, and since both in and out have frictional losses and other inefficiencies, the lighter the wheel the better. QED.

In an engine you have a flywheel; same concept but in this case you need it to smooth out the harsh peaks that come from piston compression and the resulting explosions.

IOW: go with the lighter wheels esp if the D is the same. Think about getting some cheap alloys. :cool: