markz -
I went
here and repeated your calculations. It would be a 6.6% gain with the new tires. If the odometer said 60 MPH, you'd actually be going 64 MPH. The wheels would raise the car up by 0.75". I agree that this is a cool mod.
Just to ask my standard-issue questions. Would the new tires fit on the same wheels and would the new tires fit in the wheel-well? Also, what is the before/after weight difference of the tires?
CarloSW2