How much difference in ET time between running
1) CSA 16" Ultra w/ Yokohama A520 205/40/16
vs
2) Mugen RNR 15" w/ Potenza S02 195/50/15

I was reading up on polar inertia and the idea I got was that although the Mugen rims are almost like 10+lbs lighter then the CSA's but due to the higher profile of tire and the polar inertia, ET times wouldn't be too diff

My buddy says w/ the Mugen rim ur ET time should improve by at least .2 secs. Can anyone elaborate?

 

Getting and ET difference between the 2 wheel/tire sizes will be hard. It all depends on your tire. Rember that the tire height effects the final drive ratio of your trans. the 16" rim and tire will be 19.23" tall while the 15" rim and tire will be 18.84. The shorter tire/wheel combo will have quicker aceleration but might cause you to shift to another gear at the end of the quarted mile wasting time. A taller tire will give you a bit more on the top end but will accelerate slower. There are calculations to fiqure this out but I have forgotten them all.

A lighter wil will spin easier from less mass to rotate, but you could spin too fast and loose traction and get a higher 60' time and higher ET. With a 15" rim you can get a slick with more side wall to flex under load and that will help traction a great deal. You will also have a wider tire size selection with 15" rims. Supposedly every 100lbs you save, it is worth .1 seconds and 1 MPH in the quarter.