People have a choice about how they choose to make adjustments to their vehicles. Stating that the people who drives cars with after market parts resides where education is prevelant, is foolish. I have two degrees, and currently working in the legal field. I drive a Impala with with 22 inch rims, and I also drive a Mercedes S430 with 20 inch rims. I have a relative that has a new model Mercedes S550 that is a business owner. We both reside in premier neighborhoods, not the ghettos. PLEASE STOP stereotyping people by the cars we drive. Eventually the police will too, someday…

Bear with me here I have a few things to add and some corrections for a few of the uninformed individuals.

It amazed me that no one considered the effects that varying your overall tire height has on your efficiency. If no re-gearing is done it is quite possible that you have changed the speed at which the car will be most fuel-efficient. In some cases this can help or hinder your fuel economy depending on driving style. I thought it was interesting that many of the people writing in this page assumed that putting larger diameter rims on your car will increase your ride height and distance traveled per tire revolution. Obviously you have to put a different tire size on the car and there are space limitations as to what size tire you use (rubbing issues with fender wells).

Correction: automakers don’t always design their cars with max. fuel economy as the paramount goal. They design them for sales and sometimes reliability and customer satisfaction because it is in their best interest. In most cases modifications can be made to increase fuel efficiency these are not part of the original design because of beliefs that installing the mods on the cars from the factory would produce no increase in profits.

As for decreasing wheel weight I think one person got it right by saying that decreasing moment of inertia without changing overall tire diameter or width will increase fuel efficiency.

If you are changing the overall tire height this will change the effective gearing of the car (taller tire will facilitate higher speeds per rpm). But gas mileage is not as simple as relating rpms to fuel efficiency. Powerbands and “fuel efficient bands” are almost never the same. The overall fuel efficiency at different speeds is based mainly on two independent characteristics: engine fuel efficiency characteristics and the nonlinear increase in drag with relation to speed (nobody has said a word about drag on this forum – I am talking about drag on the vehicle, not air resistance on the tires… obviously increasing the width of the tire increases air resistance and thereby decreases fuel efficiency)

Here is a quote from Wikipedia that I found useful. “Note that the power needed to push an object through a fluid increases as the cube of the velocity. A car cruising on a highway at 50 mph (80 km/h) may require only 10 horsepower (7.5 kW) to overcome air drag, but that same car at 100 mph (160 km/h) requires 80 hp (60 kW). ”

So lets say that lab testing shows that your car produces the most power at the wheel per gallon of fuel used in fifth gear at 2800 rpms and this translates to 65 mph. 65 mph is most likely not your most efficient speed to travel in a real environment with air drag. Final fuel efficiency estimates at various speeds can only be found by road testing or by coupling the results of the previous test with wind tunnel testing. For instance if this car exhibits similar power efficiency (maybe slightly lower) at a lower speed, say 45mph, it will be more efficient at a lower speed than 65 mph because of the huge effects that drag has on fuel economy.
See Link: http://en.wikipedia.org/wiki/File:Fuel_economy_vs_speed_1997.png

Changing tire height can sometimes help fuel economy if it causes the car to exist more often in the fuel-efficient zone. If you are consistently driving at high speeds (65mph +) a taller tire will most likely give you better fuel economy. However this will not be accurately realized unless your odometer is adjusted or you do calcs to correct for tire height. And this mod will not always be fuel efficient depending on the increase in drag encountered by changing ride height.

The only fuel economy increasing methods that I can think of which are always successful on any vehicle are as follows:
1. Decrease moment of inertia of the rotating wheel (w/o changing the tire height)
2. Decrease drag on the car …ie. Make more aerodynamic
3. Decrease the weight of the car
4. Installing an accurate fuel economy gauge which displays instantaneous fuel edonomy… assuming that this causes you to be more aware of when you are being fuel efficient… Research Hypermiling: http://en.wikipedia.org/wiki/Hypermiling

Note: Making mods to the cars engine can also help sometimes with fuel economy.

Your best bet is to do your research on your car and find stories of people who have tried exactly what you are doing on your particular car… unless of course you own a test lab or have an endless supply of money to try mods out and do appropriate road testing.

Correction: automakers don’t always design their cars with max. fuel economy as the paramount goal. They design them for sales and sometimes reliability and customer satisfaction because it is in their best interest. In most cases modifications can be made to increase fuel efficiency these are not part of the original design because of beliefs that installing the mods on the cars from the factory would produce no increase in profits.

As for decreasing wheel weight I think one person got it right by saying that decreasing moment of inertia without changing overall tire diameter or width will increase fuel efficiency.

If you are changing the overall tire height this will change the effective gearing of the car (taller tire will facilitate higher speeds per rpm). But gas mileage is not as simple as relating rpms to fuel efficiency. Powerbands and “fuel efficient bands” are almost never the same. The overall fuel efficiency at different speeds is based mainly on two independent characteristics: engine fuel efficiency characteristics and the nonlinear increase in drag with relation to speed (nobody has said a word about drag on this forum – I am talking about drag on the vehicle, not air resistance on the tires… obviously increasing the width of the tire increases air resistance and thereby decreases fuel efficiency)

Here is a quote from Wikipedia that I found useful. “Note that the power needed to push an object through a fluid increases as the cube of the velocity. A car cruising on a highway at 50 mph (80 km/h) may require only 10 horsepower (7.5 kW) to overcome air drag, but that same car at 100 mph (160 km/h) requires 80 hp (60 kW). ”

So lets say that lab testing shows that your car produces the most power at the wheel per gallon of fuel used in fifth gear at 2800 rpms and this translates to 65 mph. 65 mph is most likely not your most efficient speed to travel in a real environment with air drag. Final fuel efficiency estimates at various speeds can only be found by road testing or by coupling the results of the previous test with wind tunnel testing. For instance if this car exhibits similar power efficiency (maybe slightly lower) at a lower speed, say 45mph, it will be more efficient at a lower speed than 65 mph because of the huge effects that drag has on fuel economy.
See Link: http://en.wikipedia.org/wiki/File:Fuel_economy_vs_speed_1997.png

Changing tire height can sometimes help fuel economy if it causes the car to exist more often in the fuel-efficient zone. If you are consistently driving at high speeds (65mph +) a taller tire will most likely give you better fuel economy. However this will not be accurately realized unless your odometer is adjusted or you do calcs to correct for tire height. And this mod will not always be fuel efficient depending on the increase in drag encountered by changing ride height.

The only fuel economy increasing methods that I can think of which are always successful on any vehicle are as follows:
1. Decrease moment of inertia of the rotating wheel
2. Decrease drag on the car …ie. Make more aerodynamic
3. Decrease the weight of the car
4. Installing an accurate fuel economy gauge which displays instantaneous fuel edonomy… assuming that this causes you to be more aware of when you are being fuel efficient… Research Hypermiling: http://en.wikipedia.org/wiki/Hypermiling

Note: Making mods to the cars engine can also help sometimes with fuel economy.

Your best bet is to do your research on your car and find stories of people who have tried exactly what you are doing on your particular car… unless of course you own a test lab or have an endless supply of money to try mods out and do appropriate road testing.

Circumference plays a part in the mileage factor, but is often offset in that it pushes the weight that needs to be spun further from the axle, so usually you don’t see a gain or loss from that.

The best is to have a light wheel that has as much weight as it can towards the center. The closer the weight is to the axle, the easier it will spin.

Heavier tires are easier to maintain at a constant speed. Thus good for highway milage, or for driving that doesn’t stop or change speeds often.

RMP at 1,000 takes more gas than RMP at 10,000

To a practial point, the less contact area the more efficient.

Forget Newton or Gali the ideas you all are thowing around are way beyond basic physics. Various cars will have a nonlinear reaction to changes in their drivetrain.

On the whole weight issue, you just can’t take it to an extreme:

16 inch steel wheel weighs more than a 16 inch forged alloy wheel.

16 inch steel wheel weighs the same or LESS than a chrome-plated wheel, b/c the plating makes an alloy wheel heavier.

16 in steel wheel weighs LESS than a 22″ chromed-out spinning set of Dubs, b/c of the aforementioned chrome, plus the additional spinning hardware.

Even in light alloy wheels, if you go to an extremely large size, you’re dramatically increasing the weight of the wheel, which cancels out the weight benefits. Again, check out tirerack.com or discounttire.com and look at the weights of wheels rise by about 2-4 lbs. PER SIZE.

Now for the TIRES: usually when you get big rims they’re accompanied by high-performance tires that are biased towards tons of grip, not low rolling resistance…aka lower MPG.

The trick is to buy a lightweight alloy wheel, match it with a low-rolling-resistance tire and if you’re going to get a larger wheel, go up no more than 2-3 sizes (ride comfort will get shot beyond that anyway).

If you look at Honda, up until recently their high-performance versions of cars did not have big wheels…even their high-end NSX sports car had 17s (and I believe 18s in their last 2 model years), b/c you reach a point where big wheels become a heavy burden & increase unsprung weight (look it up), no matter of what alloy it’s made out of.

I’m not a follower of Newton or Galileo or Socrates or Einstein or whomever; I’ve just had the experience of putting heavy chrome wheels on my own car as well as others back in my heyday. The big wheels increased grip tremendously, at the expense of steering feel (much more numb) and MPG.

Now that I’m older (and hopefully a bit wiser), I only upgrade 1 or 2 sizes up, and look for the lightest decent-looking wheel possible (not flashy!) and match it to an all-season low-roll resistance tire.