If you’re like a lot of the Jalopnik staff, you’ve probably got a car old enough to have a dedicated overdrive button on or around your gear shifter. And if you’re like me, you probably never touch it, because who the hell knows what it actually does? Well I’m here to tell you, and then you can tell all your friends exactly what overdrive is for, when you should use it and how it works. If you ask me, overdrive is one of the coolest named functions a car can have, but it’s up there with “Roadmaster” for cool names for boring things.
Anyway, overdrive is a mechanism meant to reduce an engine’s RPM at sustained highway speeds. It’s a way to improve fuel economy and drive more efficiently at higher speeds, according to J.D. Power, and it limits the amount of strain your engine undergoes to achieve a smoother ride overall. Basically, it’s a net positive.
How overdrive works and its origins
In ye olden days of auto manufacturing, three-speed automatics were fairly common. Top gear was usually direct drive, meaning the transmission’s input shaft (which is directly connected to the engine) and its output shaft (which is indirectly connected to the wheels) would spin at the same rate. Lower gears, like first, might make the input shaft spin at three times the output shaft’s speed, leading to quicker acceleration. Second gear would be somewhere in between. The lower gears make it easier to get moving and the higher gears lead to a more relaxed cruising experience.
As fuel economy standards became more stringent, automakers began adding higher gears to make the input shaft pin slower than the output shaft, Cars.com says. It would lead to “overgearing” or “overdriving” the engine at higher speeds, and thus, overdrive as we know it today was born.
Both manual and automatic cars can have overdrive, but in manual transmission cars, it’s usually just the highest gear. In an automatic, overdrive is often just an automatic function (pardon the joke) initiated by the car’s ECU, according to J.D. Power. Of course, as newer automatics get more gears — up to 10 in some cases — the need for a dedicated overdrive has gone out the window.
When to use overdrive and when to turn it off
Generally, if your car has a dedicated overdrive button, you can just leave it alone. Your car’s ECU is smart enough to know when to engage it. However, sometimes it’s a good idea to lock out overdrive.
If you click that button, it’ll lock out the highest gear. This can help when you need your car’s full power to accelerate for passing, climbing a hill or pulling a trailer, J.D. Power says. Those tasks require a vehicle’s torque to be readily available, and having overdrive enabled stops that from happening.
Obviously, if you aren’t doing any of these things, you should really just leave overdrive on since you’re going to get better fuel efficiency and put less stress on your engine, and who doesn’t want that?
