Which has lower RR? Large or small diameter tire
 

All else being equal (I know, I know, all else isn't equal) does a bigger tire OD help or hurt rolling resistance?

Well on bicycles anyway, larger diameter wheels reduce rolling friction, so that could be inferred to car tires also. Probably something to do with the weight spread over a longer patch and less or more gradual flex of the tire.

All else being equal...hmmm.

The disadvantage is that more weight is farther from the axle meaning more energy used to spin them up and slow them down and turn them.

Might want to have a peek at this thread. EM's resident tire expert also suggests larger diameter = better.

http://ecomodder.com/forum/showthrea...tter-9484.html

What isn't clear in the thread mentioned above is that it isn't tire diameter that has the effect on RR.

It's load carrying capacity - which, if you hold everything else the same (which you can't), is what happens when you only increase overall diameter.

Here's an example of the problem:

P205/70R15: To increase the diameter, you'd have to increase the aspect ratio = P205/75R15. That also increases the load carrying capacity - which is a factor for RR.

BUT - the largest factor is the amount of rubber and the rubber compound itself. A different tire can have a much larger effect than a change in tire size (and load carrying capacity!) The problem is the RR doesn't come without a price to pay: traction (especially wet traction) and / or wear.