CarTalk.com Best of Deals Car Reviews Repair Shops Cars A-Z Radio Show

Over drive

what is over drive in a car

Overdrive refers to a special gear in the transmission that allows the engine to turn slower than the output of the transmission. A typical transmission is geared like this:

First gear had a ratio similar to 3:1, meaning the engine turned 3 revolutions to every one revolution of the transmission output. This allows the car to pull away from the stop light easier.

Second gear had a ratio of 2:1, meaning the engine turned 2 revolutions to every one revolution of the transmission output. It allows for a better transition from first gear to third gear.

Third gear had a ratio of 1:1, and was once called final drive. Until overdrive was used commonly in the late 80’s and early 90’s, most cars had this as the last gear.

Overdrive has a gear ratio like 0.75:1, which allows the engine to turn slower that the transmission output. This is also called the cruising gear, and provides for better mileage at highway speeds. You don’t get an efficient transfer of power from the engine with this gear, but today’s cars need less power to maintain highway speed due to better aerodynamics.

Overdrives were originally auxillary transmissions in tandam with the then common 3 speed transmissions that gave the car a higher top gear and they did this by “gearing up” the output of the transmissions. The driver had his 3 gears and a knob that changed the overdrive unit from direct drive to overdrive, effectively giving the car 6 forward ratios and two reverse ratios.
These overdrive units were often aftermarket add ons.

The modern “overdrive” is simply a name for the car’s highest gear which may or may not turn the transmission’s output shaft faster than the engine speed. When is your highest gear an “overdrive”? If your car has a higher top speed in 4th gear than it does in 5th gear, then 5th is an “overdrive”.

My motorcycle’s 5th gear has a 1:1.05 reduction ratio, there is no 1:1 ratio in its transmission. The pre-transmission gear reduction and the post transmission gear reduction makes the overall gear reduction 4.191 engine revolutions per rear wheel revolution. This has the engine turning about 3400 rpm at 60 mph. Wanting better gas mileage and the fact that I was always looking for that non-existant 6th gear while riding inspired me to replace the 17 tooth countershaft chain sprocket with a 18 tooth sprocket and replace the 42 tooth rear wheel sprocket with a 41 tooth sprocket. Now my 5th gear is effectively an overdrive even if it is not so in the literal sense of the word. If I ever need to go 150 mph, I will just have to leave it in 4th gear.

I have always believed that overdrive means a gear ratio where the output shaft of the transmission turns faster than the rpm of the engine. Years ago, some manufacturers offered the Borg-Warner overdrive which was a unit behind the 3 speed transmission. When the overdrive was engaged, one would release the accelerator at about 30 miles per hour and the vehicle would go into a higher gear. Nash, Studebaker, Hudson, and Packard offered an overdrive. Chrysler had the overdrive available before WW II and re-introduced it for the Plymouth and Dodge in 1952. Ford began offering the overdrive in 1949 on all its models. Chevrolet offered the overdrive beginning in 1955 through the mid 1960’s. The Willys Jeep Station wagon provided overdrive as standard equipment so that the 4 cylinder “go devil” engine wouldn’t over rev at highway speeds.

In some of the early 1970 compact models (Valiant, Duster), Chrysler offered a 4 speed manual transmission where the top gear was an overdrive. The overdrive in today’s automatic transmission is a gear ratio where the engine rpm is slower than the output shaft from the transmission.

"If your car has a higher top speed in 4th gear than it does in 5th gear, then 5th is an “overdrive”.
Are you sure about this???

If this is a rear wheel drive vehicle, overdrive is a ratio that is higher than 1 to 1 like 0.8 or 0.7 i.e. the drive shaft will turn 1.2 to 1.5 revolutions for every 1 turn of the engine crankshaft. Some rear wheel GM vehicles will have two overdrive ratios (0.87 and 0.65) after 1.00.

On a front wheel drive vehicle overdrive is a misnomer as the transmission is always reducing the engine revolutions to the axle revolutions. In some automatic transmissions the “overdrive” gives a higher ratio above the ratios given by the standard first three speeds. This gives an effective 4th speed to a 3 speed transmission.

Hope this helps explain it.

Researcher, are you sure about all that? I find it hard to believe.

He’s probably saying that the differential and transmission are combined so there is always gear reduction. Or he is saying that there is no overdrive because the engineers just made the differential to a higher speed ratio and are calling the final result “overdrive”. Or not.

Overdrive means simply that the output is turning faster than the input.

In a car the transmission usually reduces the output speed while increasing the output torque. In a manual transmission ratios of 2.0 to 2.7 to 1 are common for 1st gear. That means the input turns 2.0 times to the output turning once. In overdrive the input shaft turns once and the output turns more than one full turn, maybe 1.1 turns, etc.

On second thought you have a point there. I have been considering the ratio of the input shaft to the output shaft of the transmission. On a longitudinal transmission an overdrive is obvious. But on a transaxle the input shaft is always turning faster than the axle shafts. Yes there would be a point in the transmission where a shaft is turning faster than the input shaft and would be technically overdriven. So that may be what the engineers mean by overdrive.