O/d?

audiocr381ve · Post by **audiocr381ve** » Thu Jul 21, 2005 8:28 am

What exactly does over drive do? Boost up the RPMs? Which makes the car faster, on or off? Which saves gas, on or off?

sinister audio · Post by **sinister audio** » Thu Jul 21, 2005 8:35 am

saves gas when its on, its the 4th gear in a car well most cars. it just keeps the motor at lower rpms for using less gas and keeping th motor from running hard. you turn it off when your going up hills or towing stuff. depending on the car will tell if the car has enough power to make it go faster in 3 or 4th and how far the rpms can go.

audiocr381ve · Post by **audiocr381ve** » Thu Jul 21, 2005 8:38 am

I've heard different people say different things about it. Thats what I thought thanks dude.

Post by **GlassWolf** » Thu Jul 21, 2005 9:36 am

simply put, overdrive is a gear reduction. top gear in an automatic tranny is a 1:1 ratio. every one revolution of the flywheel is one revolution of the drive shaft. after that the gearing at the wheels is determined by the final gear ratio of the "rear end" of the car (the transaxle, when you hear someone say "4.11 gears" it means every 1 revolution of the driveshaft equals 4.11 revolutions of the axle/tire)

What overdrive does, is take that top gear 1:1 ratio and drop it for example to a 0.78:1 ratio, so every 1 rev of the crankshaft (motor) equals .78 revs of the driveshaft on the backside of the transmission.

The whole point of AOD is to improve gas milage and make the car more driveable at highway speeds by reducing the engine RPM at high speeds. at a 1:1 gear ratio with an old non-OD 3 speed tranny, you could be running at 3500rpm at 60MPH, which eats gas and wears on the engine, and causes driver fatigue from the noise and jerkiness of the engine at higher RPM cruising speeds.

Post by **GlassWolf** » Thu Jul 21, 2005 9:43 am

example:
3 speed automatic with AOD:
1st gear: 2.45:1
2nd gear 1.45:1
3rd gear: 1:1
overdrive: 0.69:1

note the transmission multiplier below 3rd gear also adds a torque multiplier as well, which I didn't mention earlier. I was trying to keep this fairly simple.

Mithrandir · Post by **Mithrandir** » Thu Jul 21, 2005 2:50 pm

alright, so whats a good speed to switch at? I usually keep it in drive until i hit 50-55 then use over drive for anything over that. Is that a good or bad idea?

Post by **GlassWolf** » Thu Jul 21, 2005 4:48 pm

you should leave the car in automatic OD, and let the transmission decide when to use the OD. The only reason you should disable OD is for climbing hills when towing a trailer to avoid constant shifting as the vehicle needs extra power to pull the added weight up mountains and hills on the freeway.

Mithrandir · Post by **Mithrandir** » Fri Jul 22, 2005 3:24 pm

I dont have automatic over drive that I know of I can pick park, neutral, reverse, od, drive, 2nd and 1st

Post by **GlassWolf** » Fri Jul 22, 2005 6:31 pm

that's automatic overdrive, Mith.
leave the car in OD. That's drive with OD enabled. The car will go into OD on the highway at crusing speeds, to save fuel.
Did you ever read your owners manual for the car? Or perhaps take driver's ed?

I honestly never thought I'd meet someone whow as confused by an automatic transmission. Do you drive a manual, too or never had a need to learn the use of a clutch and gear shift? I'm just curious, really.

audiocr381ve · Post by **audiocr381ve** » Fri Jul 22, 2005 6:39 pm

who the hell reads car manuels