When Does a Car Use Overdrive?
A car uses overdrive when it is traveling at high speed over long distances. Overdrive, or cruising gear, is a device which enables the engine to run at a relatively low speed even when the vehicle is traveling fast. All internal combustion engines fitted in vehicles need some kind of gearbox because their efficiency at low speeds is poor. The use of different gears enables the speed of the engine to be harmonized with that of the car. The gears may be engaged or shifted by hand or operated by an automatic gearbox.
Most cars have a four-speed gearbox. The driver uses first gear for starting and changes to second and third gears as the car gains speed. Finally in top or fourth gear the engine speed is transmitted unreduced through the gearbox. In overdrive a large gear wheel drives a smaller gear wheel on the propeller shaft. This shaft then rotates faster than the engine, thus reducing wear and tear and saving petrol.
Generally speaking, overdrive is the highest gear in the transmission. Overdrive allows the engine to operate at a lower RPM for a given road speed. This allows the vehicle to achieve better fuel efficiency, and often quieter operation on the highway. When it is switched on, an automatic transmission can shift into overdrive mode after a certain speed is reached (usually 70+ km/h [40-45 mph or more] depending on the load). When it is off, the automatic transmission shifting is limited to the lower gears. Overdrive should usually be selected when the average speed is above 70 km/h (40-45 mph).
The automatic transmission automatically shifts from OD to direct drive when more load is present. When less load is present, it shifts back to OD. Under certain conditions, for example driving uphill, or towing a trailer, the transmission may “hunt” between OD and the next highest gear, shifting back and forth. In this case, switching it off can help the transmission to “decide”. It may also be advantageous to switch it off if engine braking is desired, for example when driving downhill. The vehicle’s owner’s manual will often contain information and suitable procedures regarding such situations, for each given vehicle.
Virtually all vehicles (cars and trucks) have overdrive today whether manual transmission or automatic. In the automotive aftermarket you can also retrofit overdrive to existing early transmissions. Overdrive was widely used in European automobiles with manual transmission in the 60s and 70s to improve mileage and sport driving as a bolt-on option but it became increasingly more common for later transmissions to have this gear built in.
If a vehicle is equipped with a bolt-on overdrive (e.g.: GKN or Gear Vendors) as opposed to having an overdrive built in one will typically have the option to use the overdrive in more gears than just the top gear. In this case gear changing is still possible in all gears, even with overdrive disconnected. Overdrive simply adds effective ranges to the gears, thus overdrive third and fourth become in effect “third-and-a-half” and a fifth gear. In practice this gives the driver more ratios which are closer together providing greater flexibility particularly in performance cars.