Fuel Mileage Efficiency in High Gear

Ever since the gas shortage in the 70s people have considered fuel mileage as a part of their decision making a car purchase. Smaller, more efficient engines were part of the equation but not everyone considered the role the transmission played in equation.

Automatic overdrive transmissions were first introduced in 1978 by Toyota. In 1980 Ford came through with an automatic overdrive, followed by General Motors in 1981. Shortly after every auto manufacturer had a four-speed automatic overdrive available in the car line; it became a mainstay for many years.

An overdrive transmission offers the benefit of fewer engine revolutions, hence less fuel needed, per driven mile. Think of it like a 10-speed bicycle: when you’re in a higher gear, like 10th, you barely have to pedal to go pretty fast. It’s harder to pedal, and if you get to a hill you have to switch to a lower gear but while you’re cruising you can just zip along without too much effort.

An overdrive transmission offers the same advantage. In overdrive the engine spins slower and uses less fuel as you’re cruising down the highway. And just like the bicycle example, the transmission has to shift to a lower gear to get up a hill or for hard acceleration.

Beginning in the early 90’s car manufacturers began enhancing this idea with the introduction of five-speed overdrive transmissions, followed by six-speed transmissions in the early 2000s. Here, the strategy was a little different: by having sixspeeds it allows the engine to stay at its optimal rpm (revolutions per minute); where it runs most efficiently andoffers better fuel mileage.

Now, a new problem was developing: These new five and six-speed transmissions keep the engine at its optimal speed but the number of internal part required to get those speeds, in itself, became problematic. There’s a level of inefficiency built into a design with that many parts spinning around.

In 2011, the ZF corporation introduced a radical design that not only offers eight speeds for even better engine efficiency but they did it with only five friction elements (clutches) compared to eight in many of the six speeds. The ZF8-HP45 was first introduced in 2011 for select Audi and BMW cars and is now used in the 2012 Dodge Charger and Chrysler 300.

The transmission is too new to report on any problems or service issues but it’s a big jump in transmission technology. The transmission has always been an important component of a car’s performance. And as the technology continues to improve we’re sure to see more efficient models appear. There’s already news about a 10-speed on the horizon.

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