Originally Posted by
79pmooney
There's a non-math way to compare any two wheels. (Old as the hills. It's called a roll-out.) Get your bike. Put it on any flat surface with the tire valve worn. Put a piece of tape on the surface at the valve. Now roll the bike until the valve comes down again. Measure valve to valve. Repeat for your other wheels. The percent difference is your percent difference in recorded speed.
(Edit: this takes into account that tires are often not actually "42c" as labeled on the sidewall.)
If you are really concerned about being accurate, this is the way to do it. Wheels may not always be the same as the theoretical. Then as the tire wears, it will change a little. Not enough to be concerned about.