What is overdrive on a truck?

Someone will be around shortly with the detailed answer. In the mean time here is my limted information regarding something i know nothing about

If you start in first gear the tires are odviously doing less revolutions than the (flywheel?) Overdrive would be the point where the gear ratio gets to a point that the wheels (through gearing) rotate faster.

Think about a ten speed bike. In the highest gear your legs are going faster than the wheels. Overdeive would be the point the gears allowed tour legs to spin less the the revs of the wheels.


I think thats it...

 

Direct drive means that the output shaft from the engine is turning at the same speed as the drive line at the back of the transmission. In overdrive, the output shaft from the engine to the transmission will be turning slower.

 

Yeah what he said

 

Direct drive is 1:1
Overdrive is anything less than 1:1, like 0.87:1

Basically - if it's a direct drive 10 speed, then tenth gear is direct.

If it's a 10 speed OD, which most are, then 9th gear is direct, and 10th is OD.

The engine turns less RPM in OD gears, to achieve the same wheel rotation.

 

But isnt the most of efficiency achieved at direct drive?

 

Overdrive in cars and pickups is just the highest gear.
Chevy had 3 speed automatic transmissions forever, then went to the pos 700 "overdrive" transmission, which was basically a 4 speed.
Nowadays I'm sure they all have 6 speeds? Who knows? Nowadays if the windshield gets dirty, people buy a new vehicle.

But,what is this overdrive you think is in the big trucks?
We have 9, 10, 13, and 18, (plus more or less) geared transmissions.

I think you're confused, and most the answers you got before me, are complete nonsense.

 

Umm.... I'm thinking you are the confused one. Yes the 700R4 was an overdrive trans yes it was a 4 speed and 4th was overdrive. But.....
Overdrive has nothing to do with being the last gear in the trans... although most of the time it is. It's called that because the tail shaft or drive shaft is being over driven meaning it is turning faster than the engine. One gear is direct drive everything below is under driven everything higher is overdrive.
Yes direct is more efficient than over drive but only by a few percent (because of the added heat). But in the real world you are not likely to see it. It's better to be in the engines sweet spot OD helps you get there.

 

Transmissions for years were a 1-1 ratio in high gear. Anything less than the engine speed of the transmission tail shaft is an over drive. Overdrive will only get you better fuel mileage if your engine has the power (sweet spot) to run at that rpm.

 

Depends on your transmission. Like was stated above if you've got an overdrive transmission the top gear will be overdrive. On 13 and 18 speed overdrive transmissions I believe the top 2 are overdrive (8th hole in high range will be overdrive but you can split that gear so you'll have 2 overdrive gears.