I would think you would want to account for you tire size when you are doing this too. Without putting the tire size into the equation, you cannot get an accurate final reading of what your rpm's will be. There are all kinds of rpm calculators on the interwebs. They figure out your engine rpm based on tire size, transmission gear ratio, and rear-end gear ratio. I did a bunch of this when I changed out my 3 speed for a 5 speed on my car.
