And I came to the realization that the OEM 17 and 18 wheels (coupe) don't have the exact same diameter.....which is news to me. I didn't expect them to be exactly the same, but a difference of nearly 2 km difference being registered by rear wheels of a 17 inch and 18 OEM wheel?

Does that mean that 3 stock G35 coupes...one with 17 inch wheel, one with 18's, and one with 19s would all register slightly different speeds on their speedometers when driven perfectly side by side? A little confused here....

Btw, the tire size calculator: http://www.1010tires.com/TireSizeCalculator.asp

Yes and the difference will depend on the size tires used by each. Il like this calulator too-



http://www.miata.net/garage/tirecalc.html

Unless they came from the factory like that in which case the differences would be compensated for.