[CapriRacer]

Originally Posted by Andyman

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I've read that rolling resistance is inversely proportional to tire diameter.

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Unfortunately, that is not true. At best you could say this is directional.

It is true that that tires with larger load carrying capacity generally have lower RRC (Rolling Resistance Coefficient) - and that should translate to lower RR for the same load (this is far from clear to me.) - and that larger capacity tires are generally larger in overall diameter. But this chain of connections means that there are too many variables to be able to state this as a proportion.

It is also not clear from the literature available that at a given load, RR goes down as tire diameter goes up. This seems to be right, but getting my arms around this has been elusive.

Originally Posted by Andyman

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This means that a larger diameter tire will require less power and will improve gas mileage, assuming that engine RPM stays the same.

?????? How can the engine rpm stay the same if you change tire diameter - unless you change gearing?

Originally Posted by Andyman

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So, it is probably best to install the largest diameter tires that will fit the car. The aerodynamic drag may go up because larger tires makes the car higher, so the fuel economy benefit may be reduced at high speeds. This could be adjusted by changing the springs. The reduction in engine RPM with larger tires is also likely to improve fuel economy on the highway. It is probably best to increase the rim size instead of the tire height for best handling. I'm not sure if there is a best tire aspect ratio but some low drag tires have an aspect ratio of 65%.

From what I have been able to grab, the larger the load carrying capacity, the lower the RRC - and while I agree that probably larger diameter tires would have lower RR, I think that high aspect ratios (70, 75, etc.) will probably result in better fuel economy compared to low aspect ratio/same diameter tires.

So I am going to directly contradict what Andyman said. Do not go larger in rim diameter if you want to improve fuel economy (BTW, we haven't yet addressed the larger moment of inertia of a larger diameter rim and that alone is reason enough not to go there.)


So to try to answer Guitar's question:

I'm hoping this was a mistake: 12" rim / 20 inch tire vs 14" rim /24" tire. That means the tire had to go up in aspect ratio.

AND:

The rpm of 3000 for the 20" tire would result in an rpm of 2500 for a 24" tire. (not the 3200 stated in the question)

You should get better fuel economy with the larger diameter tire based on the gearing change alone. The tire's RR will probably be better, too!