In the automotive world, O/D stands for overdrive, which signifies the top gear or gears in a car’s transmission. The O/D off button allows you to prevent an automatic transmission from shifting into those top gears in certain situations, such as when driving through rolling hills, going down steep grades and hauling a heavy load or trailer.

Related: What Is a Dual-Clutch Transmission?

But why would you want to do that? Further explanation might best start with a bit of history.

From the 1960s to the early 1980s, three-speed automatic transmissions were common. Their top gear — 3rd — was usually direct drive, which means the transmission’s input shaft (which is indirectly connected to the engine) and its output shaft (which indirectly connects to the wheels) would spin at the same speed.

By contrast, 1st gear might have had the input shaft (and thus the engine) spinning at three times the output shaft’s speed — which made for quicker acceleration — while 2nd gear would be a ratio in between. This is the same concept as on a 10-speed bicycle: The lower gears make it easier to get moving, while the higher gears make for more relaxed cruising.

As fuel economy became more important, manufacturers began adding a higher 4th gear that made the input shaft spin slower than the output shaft. This would leave the engine running more slowly — and more economically — at higher speeds, and this 4th gear was called overdrive. Other benefits included reduced engine noise and wear.

This worked fine at highway speeds on level roads. But the higher gear would sometimes overtax the engine on uphill grades, in which case the transmission would have to shift down to 3rd gear to make it up the hill.

And this worked fine until you got to rolling hills. The transmission would have to shift down to 3rd gear to get up the hill, then would shift back to 4th going downhill, then back to 3rd to get up the next hill. This back-and-forth was not only annoying for the driver (and any passengers), but it also didn’t do the transmission any good.

Furthermore, on a long downhill run, using 3rd gear would slow the car some due to engine braking, which reduced the need to use the brakes to keep the car from building up too much speed.

As a result, some automakers have added the O/D off button to allow the driver to lock out 4th gear and hold the transmission in 3rd. This can be advantageous for acceleration, even on fairly level ground when carrying heavy loads or towing a trailer.

Later automatic transmissions with five or more speeds may have more than one overdrive gear ratio, so hitting the O/D off button may lock out more than just the very top gear.

How should you use this button? For best fuel economy under normal driving conditions, you want to remember to hit the O/D off button again (there’s often a light to remind you it’s turned off) in order to allow the use of the overdrive gears.

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