* Drive (D): This is the normal driving range. The transmission will automatically shift through all the forward gears, including the highest gear available *except* overdrive. This means the engine will run at a higher RPM at highway speeds.
* Overdrive (OD or sometimes marked with a small "O"): This engages the highest gear ratio in the transmission. This is a gear designed for fuel economy at higher speeds. It allows the engine to run at lower RPMs for a given speed, resulting in better fuel mileage and quieter operation. However, it may not be ideal for towing or heavy acceleration. Some trucks had a button or switch to manually engage or disengage overdrive; others might automatically disengage it under heavy loads or low speeds.
In short, "Drive" is a more direct gear range, while "Overdrive" is a fuel-saving, higher gear designed for highway cruising. You'd typically use Overdrive for highway driving and Drive when you need more power or engine braking (like going downhill or towing).
