The transmissions have different internal parts, gears, that determine the speed for each gear at a fixed rpm. lighter cars, corvetts, use to have optional 2. 20 first gears and larger cars had (I thinK ) a 2.54 first gear, With a close ratio you would want a lower geared rear end, with a close ratio the RPM didn't change as much between shifts keeping the engine in the max power band. If you have a close ratio in a truck, with a higher geared rear end ( In the 2.xx's) you will have trouble hauling a heavy load with out burning the clutch . With 4.56 rear gears , a 2. 20 would have enough torque to pull. but you would be running higher RPM's at highway speeds.