to what extent is the performance of the car going to be affected going from 18" rims to 20".
i want to lessen the gap between the tire and wheel well.

 

Increasing the rim size will not affect the gap unless you get larger diameter tires.

If you want a smaller gap the usual solution is to lower the car with shorter springs.

 

yep, if you get 20" rims, the sidewall shrinks, not the gap. and it definitely hurts performance... quite a bit.

 

Sounds more like two mods being considered. Bigger wheels (heavier, more rotational inertia) and taller tires (lower effective overall gearing). You will lose some performance, but without more information (including just which measure of performance that you're concerned about) it is not possible to give a better answer than the method below.

I have heard that 100 lbs on the chassis is worth about a tenth in the quarter mile. So ten lbs of chassis weight would be worth a hundredth. Wheel/tire rotating weight is worth about half again (not the ten times that you might have heard elsewhere).

5 lbs wheel + tire weight difference per wheel x 4 wheels x 0.010 second per 10 lbs x 1.5 = 0.030 seconds ET difference. Scale that up for whatever weight difference corresponds to your particular case.

The tall tire effect might be worth 0.200 to 0.300 or even more depending on how big you go, unless it solves a current case of excessive wheelspin.

Maybe plan on your quarter mile times going up by at least a quarter of a second, maybe half a second?


Norm

 

yup listen to norm