Here is a link that will allow you to figure the differents in gears, tires and trannies. Tires size plays a big part in the overall ratio. You have to decide what you want to do, fast out of the hole, low rpms on the freeway. Or a compromise. Quick away from the red light is good
I chose a 5 speed to give me a little of both worlds.
http://www.secondstrike.com/Technical/GearCalc.asp
Dwight