Tire Size-Does it change fuel economy?
 

I'm an engineer, I should be able to figure this one out (it's not complicated). But my brain's tired and I'm an E.E.; not a statics/dynamics guy.

I've heard both stories.


I know the RPMs will be lower at the same speeds with a bigger tire size. But doesn lower RPMs equate to better fuel economy. We've got lots of torque so I wouldn't think a size bigger would hurt anything except acceleration.

Technically, it depends on the type of driving. For instance, a taller tire would do better in constant speed conditions whereas a shorter tire would probably be better for stop and go. It's a matter of overcoming the torque of the actual tire. Taller requires more torque to accelerate, but once at a certain speed, it will have less RPMs per mile.
Two other factors probably contribute more than the slight height difference. One is the width of the tire. More rubber on the ground, the more drag. The other is related as the more air pressure (to a point) lessens the rolling resistance.

It depends on the tire/wheel you select. I went to slightly wider 16's with the same rolling diamiter as the stock setup. I noticed no change in performance or fuel mileage.


But if you go to something larger like 18, 19, or 20's you will notice a drop. Also your brakes will have to work harder, which isn't really a problem on MB's but on normal cars it is.

I am not too worried about performance drop. My 300d is plenty fast as it is. I have to go easy on the pedal so it doesn't get off ground when the boost kicks in;) I wouldn't mind it being a little slower off the line.

Never thought I'd say I want my car to be slower!

So will lower RPM's equate to less fuel consumed?

I'd assume to maintain a given speed there's a balance of torque/RPMs. Lower the RPM's; raise the torque required by the engine. This seems to me that it's still require a similar amount of fuel since even though the engine is turning slower it's still feeding itself enough fuel to keep the higher torque.

With a diesel, fuel economy always benefits from lower engine speeds. The fuel consumed to do the work remains relatively constant and the fuel required to overcome engine friction is reduced.

There are exceptions if you push this philosophy too far, but, with regard to larger tires, they always provide better overall fuel economy in an automotive diesel.

So with the word ALWAYS established... that's a word I like, thanks Brian, what kind of changes would be gained through a 5% increase in tire diameter size while maintaining the same width?

I'd guess that the 5% increase in diameter would yield a 3% difference in overall fuel economy.........a bit less in the city.........a bit more on the highway.

We had an intense discussion regarding this topic. There are some who will swear to you that you'll get the full 5%. They would be incorrect, however.

Some would say nearly the full 5%, depending on conditions.

Tom W

I would think it they would reduce fuel economy slightly.

For arguments sake 18in AMG's weigh a lot more than the stock 15 holes, and you need energy to get that mass spinning.

The heavier wheels would slow acceleration a bit but constant state should be better.

When doing it with tires, though, with the error in the speedometer, it will appear that mileage is less, but after correction it will be more.

Tom W