how does over drive work?

Answer 1

The food pellet drops down and the little hamsters on that wheel next to your engine start running.

Answer 2

I'm not an engineer, just a simple old farm boy who has run a lot of different tractors and equipment. So let's start with the assumption that an engine runs most efficiently at a certain rpm (revolutions per minute). Let's use 2,500 rpm for an example. However you have to start moving the vehicle at a lower rpm than 2,500, when the engine may not have enough torque (power) to move the load. That's where the transmission and rear end gearing comes in. Let's forget about the rear-end gearing and assume that your drive wheels turn at the same speed as your drive shaft.

Your transmission allows you to start out in first gear and progress through the gears until you get to cruising speed. Which in this example would be when your engine is turning 2,500 rpm.

For the sake of discussion, let's say in first gear your drive shaft will revolve at a 1 fourth to 1 ratio compared to engine speed. So at 2,500 rpm engine speed, your driveshaft may only revolve 625 rpm. This allows you to utilize the power of your engine and differing rpm's to smoothly get your vehicle or load moving and to accelerate faster. Taking this a step further, second gear may be a 1 half to 1 ratio and your wheels and drive shaft would turn at 1,250 rpm in second gear when the engine is turning at 2,500 rpm. Say that third gear is a 3/4 to 1 ratio so the wheels would be turning at 1,875 rpm when the engine was running at 2,500 rpm. In my youth, your highest gear was usually fourth and it was direct drive, or a 1 to 1 ratio. In that gear your wheels would be turning one time for every turn of the engine. In my example, 2,500 engine rpm would produce 2,500 wheel rpm.

To improve fuel economy the auto makers started adding an extra gear, or as we called it back then, an "overdrive" gear. So in my example, the ratio of the overdrive would turn your driveshaft and rear wheels 1.1 or 1.2 times for every revolution of the engine.

Now we are seeing six speed manuals with maybe a couple of overdrive ratios. To confuse you even further, my old pick-up truck was a 4 speed with a "granny" gear which I guess you would call an " extreme underdrive" as it was designed to help start heavy loads. You would start that truck out in second gear under normal driving conditions.

And automatic transmissions are designed to accomplish the same thing as the manul transmissions do. I think I've seen a car advertised with an 8 speed transmission. The new pick-up truck I want has a 6 speed transmission.

I hope this was written clearly enough.

Answer 3

Generally, actually in every case I can think of, your "drive" gear is a 1:1 ratio. That means that ofr each revolution of the engine crankshaft you get one revolution of the transmission output shaft.

Overdrive gears give you a higher ratio than 1:1. Maybe something like 0.8:1. Meaning that every time the engine crankshaft makes a full rotation your transmission output shaft will make more than one full revolution.

Example: For sake of this explanation say your wheels/tires made one full revolution every time your transmission output shaft made a full revolution. or the ratio was 1:1. That means in drive for 8 revolutions of your engine, your car would move the distance of 8 complete tire rotations. (that distance would vary depending on tire circumference)
Now if you shifted into overdrive and your overdrive ratio was 0.8:1 in the transmission. For every 8 revolutions of your engine your transmissions output shaft and your wheels would make 10 complete revolutions. So you would cover the distance of 10 full tire revolutions or an increase of 20% in terms of distance traveled with the same amount of engine revolutions.

The way it saves fuel is that when the load on the engine is not too great, the vehicle is able to cover the same amount of ground using less fuel to turn the engine over. Or cover more grouns using the same amount of fuel. However you prefer to think of it.

Answer 4

Overdrive is simply another gear. It 'works' just like any other gear. The name 'over drive' simply refers to a tall gear ratio which allows the output speed of the gearset to be higher than the input speed from the engine. Your car is supposed to operate in over drive at high speeds; it is harder on your car not to use overdrive (higher engine speeds).

Answer 5

Overdrive is gearing in the transmission that alows the engine turn at a lower RPM at higher speeds.