A transmission certainly seems simple enough when you’re in your vehicle: put the shift lever in Drive and away you go. But that lever is attached to an extremely complex component that can affect how your car performs and what fuel economy it can achieve.
“You have an engine that’s exerting power, and you have to put that power to the ground through the wheels,” says Hayato Mori, manager of product planning for Honda Canada. “You have to put gearing in between to efficiently transmit the power from the engine to where you want it to go.”
Just like on a multi-speed bicycle, the various gears in the transmission are different sizes. By selecting the appropriate gear — either by the transmission itself on an automatic, or by the driver with a stick shift — maximum power is directed to the wheels while using as little engine power as possible. Sometimes the engine will have to work harder, such as when you’re taking off from a stop, but at higher speeds the transmission will allow the vehicle to cruise along while using relatively little power, which helps save fuel.
Transmissions usually contain anywhere from four to six forward gears (along with neutral and reverse), and some automakers even offer seven or eight.
“By putting more gears in between, you change the ratio, so it needs less engine power,” Mori says. “Let’s say you’re going up one floor, and the stairway has five steps. You go up another that has six steps. Going up five steps requires more work than six steps, because the six steps are smaller. That’s the same kind of thing you have on a transmission.”
Even so, there is still much that engineers can do with fewer gears.
“Even with a five-speed, you can change the ratio,” Mori says. “On a flight of stairs, you can vary the height or length of the steps so it’s not as energy-exerting. It all hinges on how well the technology works. We have five-speed transmissions that can achieve the same or better than some competitors with six-speed transmissions.”
Adding more gears also adds complexity and weight, Mori adds, and automakers must carefully balance the engine and transmission technologies to achieve maximum fuel economy.
• Not all transmissions have gears. Many automakers offer a continuously variable transmission, or CVT, that uses pulleys and a belt or chain to achieve a theoretically infinite number of ratios.
• A transmission’s greatest enemy is heat. If you tow a trailer or frequently carry heavy loads, make sure to have a transmission cooler installed if your vehicle didn’t come with one.