If you assume that most of the mass in the wheel is in the rim not the spokes, no doubt some may argue against this, then the 20s are killers for the 1/4mile so get 19s. The moment of inertia for a wheel will be that for a cylinder and all other points being equal the difference of the squares in the radius is the only difference. Therefore for 19s the radius is 9.5 and that squared is 90.25 but for 20s the radius is 10 and therefore we end up with the moment for 20s being ~10% more difficult to get going than 19s! It's about 19% for 18s. So, 19s better for performance but 20s look better. Yes the overall rolling diameter is the same depending on profile but I'm sure the mass of the wheel is the key to the problem not the tyre. All of this is from my long distant memory so may be wrong in several ways Cheers Geoff