Join Date: Feb 2009
Location: SF Bay Area -- Silicon Valley
I have a set of 15/16 with R888 for track on my Elise, but stock wheels and tires for street. Since they are used differently, I can't give you direct comparison on the Elise. But the physics is against a heavier tire/rim combo with the mass farther from the hub. The larger and heavier rim/tire has a higher moment-of-inertia (flywheel effect) and is harder to change direction, slower to accelerate, etc. None of that is good.
With the slightly undersized tires, the speedometer error increases from "normal" +7% to about 10% over.
On my BMW E30 M3 race car, I once used +2 (17") and the difference was very noticeable. Slower to turn in and accelerate. I got rid of that combo right away.
On another race car, I fitted slightly oversize tires (Mich Pilot Sport Cups) on standard rims. and compared to the standard sized tires (Hoosiers) they were again heavier, slower to accelerate and turn-in. And again required speedo recalibration.
It's just physics.