Slick size

It's the diameter of the tire you use in the equation, the width means nothing for the rpm calculation. I have a 28" tire in the stock wheel tubs in my car and it is to tight for a street driver. If you use a 26" tire and say you went thru the lights at 110mph and you wanted to limit your rpm to 6000 you would run a 3.91 gear. With the same tire size but only 100mph thru the lights you could get away with a 4.30 gear and stay below the 6000rpm.

Here is a calculator
http://www.moparts.com/Tech/tech/index.html


Chuck