If we all drove vehicles that were the same physical size that never varied in weight, we could probably go with the simple math approach. As it is, vehicles have been (and continue to be) manufactured with everything from a two speed Powerglide to 6-speed sticks. Trucks have long had multiple options for transmissions and rearend ratio with the goal being optimization of engine performance (a relatively small range) over a wide variety of driving conditions (a vast range). An engine runs at its best within a certain RPM range. By using a transmission, the engine’s power is delivered to the drive wheels while keeping the engine within that range. As such, various gear combinations/rations are required.

While OD may seem superfluous, it allows great fuel economy when driving conditions allow long periods of constant driving at a lower RPM. Corvette 6 speed ratios are .74 for fifth and .50 for sixth. Granted, the Corvette has a lot of power being transmitted, but the principle applies from a Smart Car to a 80,000 pound semi.

Regards,
Glenn