If you want to find the max speeds in the other gears:
All you have to do is to substitute the gear ratio for this number and recalculate....
MPH= (6000 x 23.972) / (3.14 x 4.14 x 336)
Change the number 3.14 to the new ratio..
Example: For 2nd gear...
MPH= (6000 x 23.972) / (2.52x 4.14 x 336) = 41mph
This is what you generally learn in your Physics and math classes. I couldn't recommend a specific book, but look for a physics text.