Prior to the 1980s, most domestic cars and light trucks had transmissions with a 1:1 ratio in high gear, which means that 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, auto manufacturers began to look at overdrive transmissions as part of the solution.
With an overdrive transmission, the top gear has a ratio of 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. Overdrive Typical in a domestic car is around 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) speed at 70 MPH drops to 1925 RPM That’s 825 less RPM, a reduction of almost a third.
This reduction in engine speed has several advantages:
1.) Less 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 – Other things being equal, your engine theoretically has a life consisting 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) spin at lower RPMs and should last longer.
5) Less Cabin Noise – An engine spinning at a lower RPM will be quieter, making the journey less stressful. 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 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 highway fuel economy than fuel injection and computerized engine controls. There are a number of 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 automakers didn’t offer them sooner.
