Prior to the 1980s, most domestic cars and light trucks had transmissions with a 1:1 ratio in high gear, meaning the car’s driveshaft will rotate 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 looking at overdrive transmissions as part of the solution.
With an overdrive transmission, the top gear has a ratio less than 1:1, which means the driveshaft will rotate at a faster speed than the engine. For example, if you have a car without an overdrive and a maximum gear ratio of 1:1, an axle ratio of 3.08, and a 26″ tall tire, the engine speed at 70 MPH is approximately 2750 RPM. typical in a domestic car is approximately a 0.70:1 ratio, which means that in top gear, the driveshaft will turn 42.9% faster than engine speed (1 divided by 0.70). = 1,429) The speed at 70 MPH is reduced to 1925 RPM!
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 highway, your engine will emit about a third less pollution.
3.) Longer Engine Life: All things being equal, your engine theoretically has a lifespan that consists of a certain number of revolutions. You’re going to go the same distance as before, but using less of those revs to get there.
4.) Longer Accessory Life: The water pump, alternator, power steering pump, A/C compressor, and smog pump (if equipped) all spin at lower RPMs and should last longer.
5) Less noise in the cabin: An engine that spins at a lower RPM will be quieter, making for a less stressful ride. It’s easier to have a conversation and you can listen to the radio!
However, there are some minor tradeoffs. The engine will have less power to pass and climb hills when the transmission is in high gear, so downshifting will sometimes be necessary. 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 highway fuel economy than fuel injection and computerized engine controls. There are a number of companies like Keisler Engineering that have done a brisk business providing overdrives to fit classic muscle cars and street rods! Given the advantages of overdrive transmissions, my biggest question is why didn’t automakers offer them sooner?