The ins and outs of your car's transmission system

The best automatic transmission is one the driver doesn’t notice

A V6 engine attached to an eight-speed automatic transmission. Inset: The inside of Chrysler’s eight-speed automatic transmission.


A V6 engine attached to an eight-speed automatic transmission. Inset: The inside of Chrysler’s eight-speed automatic transmission.

Whenever you put your vehicle in gear, you’re engaging a vital link between its engine and its wheels. Whether you have a stick shift or an automatic, you’re not going anywhere without a transmission.

“The transmission transfers and multiples the torque from the engine to the driving wheels,” says Jeff Lux, vice-president for powertrain transmission at Fiat Chrysler Automobiles. “If we only had a shaft between the engine and the wheels, you could only go so fast because the engine has a limit.”

Engines make their power when internal pistons move up and down to turn a central crankshaft. Coupled to the transmission, the crankshaft’s spinning motion eventually turns the wheels. That motion is measured in revolutions per minute, or r.p.m., the number indicated on the tachometer in the instrument cluster. The crankshaft spins faster on acceleration, while the transmission’s lower gears transmit maximum power to get the vehicle moving. On the highway, the transmission’s higher gears maintain the vehicle’s speed while the crankshaft slows down, which improves fuel economy.

As you slow down or speed up, an automatic transmission selects the best gear to keep the engine at its optimum r.p.m. With a standard transmission, the driver has to pay attention to clues, including the tachometer and the engine’s sound, and shift to the appropriate gear.

Putting the transmission in Neutral disconnects it from the engine, so that even though the engine is running, no power is being sent to the wheels. The Park position on an automatic actuates a mechanism that locks the park gear and prevents the vehicle from moving.

While some automatic transmissions had only three speeds right into the 1990s, six-speed units are now among the most common, while some of Chrysler’s have eight or nine speeds. While a transmission with more gears also tends to shift more often, the engine’s speed doesn’t rise or fall as dramatically during the gear changes, so the process can feel smoother to the driver.

One drawback is that, on an incline, a transmission with numerous gears may shift too much as it tries to keep the engine in its sweet spot, commonly called “gear hunting.” Engineers have to program the transmission’s software to try to avoid the problem.

The best automatic transmission, Lux says, is one that’s designed so the driver hardly notices what it’s doing. “When it’s shifting, people want the transmission to work and forget about it,” he says. “It’s like when your refrigerator is cold and you don’t think about it twice.”

Gears, etc.

• Continuously variable transmissions, or CVTs, are automatics that use pulleys and belts instead of gears.

• Automatic transmissions contain oil and filters that should be periodically changed to help ensure a longer lifespan.

• Manual transmissions have gears that slide when the shifter is moved, while automatics use gear sets that turn inside the transmission.

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