Before the 1980s, most domestic cars and trucks had transmissions with a 1: 1 ratio in high gear, which means that the car’s driveshaft rotates at the same speed as the engine. This 1: 1 ratio served us well for fifty years or more. As oil prices rose and the country as a whole became increasingly concerned about the amount of air pollution coming from our vehicles, automakers began to consider overdrive transmissions as part of the solution.
With an overdrive transmission, the top gear has a ratio of less than 1: 1, which means that the driveshaft will rotate at a faster speed than the engine. For example, if you have a car with no overdrive and a 1: 1 top speed ratio, a 3.08 axle ratio, and a 26 “high tire, the engine speed at 70 MPH is about 2750 RPM. Overdrive typical in a domestic car is about 0.70: 1, which means that in the highest gear the driveshaft will rotate 42.9% faster than the engine speed (1 divided by 0.70 = 1.429 The speed at 70 MPH is reduced to 1925 RPM! That is, 825 RPM less, a reduction of almost a third.
This reduction in engine speed has several advantages:
1.) Lower Fuel Consumption – On the highway, your engine will use about a third less fuel.
2.) Lower Emissions – On the road, your engine will emit about a third less pollution.
3.) Longer Engine Life – All things being equal, in theory your engine has a life consisting of a certain number of revolutions. You are traveling the same distance as before, but using less of those revs to get there.
4.) Longer Accessory Life – Your water pump, alternator, power steering pump, A / C compressor, and smog pump (if equipped) are spinning at lower RPM and should last longer.
5) Less noise in the cabin: An engine that turns at lower RPM will be quieter, making the trip less stressful. It’s easier to have a conversation, and you can actually listen to the radio!
However, there are some minor trade-offs. The engine will have less power to pass and climb hills when the transmission is in high gear, so it will sometimes be necessary to downshift. Most overdrive transmissions are also slightly heavier than their non-overdrive counterparts, but this difference is negligible in most cases.
All in all, overdrive transmissions have been one of the biggest improvements made to domestic cars in the last thirty years. They have made a bigger difference in fuel economy on the highway than fuel injection and computerized engine controls. There are several companies, like Keisler Engineering, that have done a good business providing overdrives to fit classic musclecars and street rods. Given the advantages of overdrive transmissions, my biggest question is why the automakers didn’t offer them sooner.