Drive/Overdrive

[Kraut187]

Is there any difference between the two, Any performance or gas mileage differences?

[Bliggida]

One is third gear, one is fourth gear. But in some instances it only controls the lockup converter. Which may only drop your RPM by about 500. Either way, it will definately help your gas mileage.

[Kraut187]

So Drive would help my gas Mileage?

[Bliggida]

Overdrive will help mileage.

[TurboKillaZ]

I started driving in overdrive and it pulls much faster than in D [img]smile.gif[/img]

[Kraut187]

Thanks guys, I know it sounded like a silly question, but who knew? I've always kept it in Overdrive, but always wondered, why is that D there. If it's not going to help any though, screw it. [img]smile.gif[/img]

[carldev96]

Overdrive Explained…

you guys need to understand what overdrive is. [img]tongue.gif[/img] Basically our automatic transmissions have 4 gears: OD, D, 2, 1. Placing the shifter in any one of these positions limits the transmission from shifting any higher. If your in D, the car will not shift into 4th gear or OD.

1st gear is something like 3:1. Thus if your engine is spinning at 3000RPM, the drive shaft will be spinning at 1000RPM. In D, or 3rd gear the ratio is 1:1. Here the engine and drive shaft spin at the same speed. The overdrive gear gets its name from the fact that the drive shaft is overdriven, or spins FASTER than the engine. I’m not sure of the exact ratio, 0.8:1??? but if the engine is at 3000RPM then the drive shaft spins at something faster (giving you a higher top speed)

Basically to get good gas mileage, you want your car to run at low RPMs. If you did not have 4th gear as an overdrive gear, doing 75 or 80 on the highway would mean running the engine at 4000+ RPM which uses more gas and puts considerable more wear on the engine and transmission. Your car pulls harder in D than OD simply because D (3rd gear) is a lower gear.

For the sake of this argument, lets just assume that our engines make a set unit of power over a single revolution. Since the engine spins 3 times faster than the drive shaft, the engine produces 3 units of power to get the drive shaft to spin once. As you can see, this puts considerable power to the ground, but you quickly hit the red line before spinning the drive shaft at any good cruising speed.

In 3rd gear, the engine only has time to generate 1 unit of power before the drive shaft has made one revolution. This puts much less power to the ground, but allows the car to pull over a much larger range of speeds. In 4th gear, (OD) the engine does not even complete a full revolution before a single turn of the drive shaft. On some instances, only 5 cylinders will have fired before the drive shaft completes a turn.

This is how you save fuel. Since creating power uses gas, its easy to see that to make the drive shaft spin once(which equates to some distance traveled), 4th gear uses the least amount of power and thus the least amount of fuel.

Sorry about the lengthy rant. Hopefully someone can some along and correct me if I am wrong about any of this. Also if someone has the real numbers for drive ratios for each of the 4 gears hopefully they would be kind enough to post them, as I am interested in this information as well.

Hope this helps a little
later
carldev

[key2emptiness]

i have had mixed reactions with it too. around town i drive in D, cause to me, it goes higher in the RPM range and with an auto that is hard to do. on the highway or freeway, i leave it on OD. is what im doing bad?? will i wear my engine faster?

[Bliggida]

You're not listening. It was just explained. OverDrive is fourth gear. If you run around town only in third gear you will waste more gas and cause more wear on the engine.

The only thing I'd add is that OverDrive also controls the lockup converter which will drop the RPM by about 500, for highway cruising.

[darkthoughts5000]

getting a little off subject here... but then, what the hell is the purpose of "1" and "2"??? i mean, i doubt the purpose GM engineered it was to just "waste" gas and produce non fuel-efficiant vehicles.

[ChronoGN]

<blockquote>quote:</font><hr>Originally posted by darkthoughts5000:
getting a little off subject here... but then, what the hell is the purpose of "1" and "2"??? i mean, i doubt the purpose GM engineered it was to just "waste" gas and produce non fuel-efficiant vehicles. <hr></blockquote>

Gear 1 and 2 is for driving in snow.

[Jason McCallister]

<blockquote>quote:</font><hr>Originally posted by darkthoughts5000:
getting a little off subject here... but then, what the hell is the purpose of "1" and "2"??? i mean, i doubt the purpose GM engineered it was to just "waste" gas and produce non fuel-efficiant vehicles. <hr></blockquote>

Or Solo-II Autocrossing! :D

[Loochy88]

sorry, but how many times has this been explained :rolleyes: :D

carry on [img]graemlins/dunce.gif[/img]

[F1GT]

<blockquote>quote:</font><hr>Originally posted by darkthoughts5000:
getting a little off subject here... but then, what the hell is the purpose of "1" and "2"??? i mean, i doubt the purpose GM engineered it was to just "waste" gas and produce non fuel-efficiant vehicles. <hr></blockquote>

Engine braking.

[F1GT]

<blockquote>quote:</font><hr>Originally posted by key2emptiness:
i have had mixed reactions with it too. around town i drive in D, cause to me, it goes higher in the RPM range and with an auto that is hard to do. on the highway or freeway, i leave it on OD. is what im doing bad?? will i wear my engine faster?<hr></blockquote>

I doubt your tranny even has a chance to go up to 4th(OD) around town. The highest gear I get up to around town is 4th, which is like 3rd for an auto. When I drive an auto I leave it in OD. Don't have to hassle with it when entering the freeway. Leave it in D if you wanna romp on it(race someone), otherwise just leave it in OD.