Velocity (ft/s) = (rpm of engine x TIRE radius x 2 x 3.14)/(dif ratio x ratio for 5th gear x 60 sec/min) so just make your dif ratio "x", solve, using what the ratio of your current transmissions 5th gear is, and how fast you want to be going at that RPM. Remeber its in ft/s for velocity. pro tip: 60mph = 88ft/s. if you are clueless about your 5th gear ratio, MAZDATRIX for Mazda RX7, RX8, Rotary Engine Parts and Accessories has a great layout of OEM transmission & dif ratios for the 1979-1995 american RX-7