OD off means that the transmission is in drive (D = 4th) and has a gear ratio of 1:1. With OD off, say at 60mph your tach is reading 2000 rpm, that means both the driveshaft and engine are spinning at 2000rpm; i.e. the 1: 1 gear ratio.

So assuming you still have your stock rear end gear, that means your final drive ratio would be 1 x 3.31= 3.31

These cars should start up with OD on meaning no-light on the dash. That puts it into the best situation for highway driving and fuel economy. Personally, I don't like the way the engine lugs in OD at around 45, so I turn OD off when I know I am going to be poking around at speeds less than 45-50 mph. I also turn it off at the stop light just in case someone feels froggy and needs a can of WA opened on them, but that's another story....

When OD kicks in (OD = 5th), the gear ratio of the automatic transmission is then 0.75:1 and your final drive ratio would be 0.75 X 3.31 = 2.4825.

What that means is this. Lets say at 60 mph in D (4th) your tach reads 2000 rpms. With OD on the tach will read 2000 X 0.75 = 1500. In addition, this means the driveshaft is spinning 2000 rpms, while the engine is only spinning 1500rpms. This lets the car use less fuel.

OD is great on the highway at a good steady speed. IMHO, in the city where you're not going over 50mph, turn it off. The car will respond better, plus you don't lug the engine around nor does it "hunt" gears all the time.

Hope this explains it and helps.

Mike

 

Is 0-60 mph faster with overdrive off?

 

close but not the computer its trottle pressure inside your tranny.. throttle pressure overcomes governer pressure affecting the shift valves its working.. then downshifts.. when governerpressure overcomes throtle pressure (say your foot is still to the floor) at high rpm, you then get an upshift

 

i thought 4th gear wasnt an EXACT 1.1 but it was slightly off like .92 :1

like its a slight overdrive.. also ive never heard of an automatic having a perfect 1:1 in 4th

standards.. yes.. 4th always 1:1 ratio

 

Here are the specs for the automatic and manual transmissions in our cars:

TRANSMISSION

Standard Five-speed manual GT = (Tremec 3650) Five-speed manual V6 = (Tremec T-5)

Gear ratios

GT V6

1st 3.34:1 3.75:1

2nd 2.00:1 2.19:1

3rd 1.32.1 1.41:1

4th 1.00:1 1.00:1

5th 0.68:1 0.72:1

Final drive 3.55:1 3.31:1

Optional Five-speed automatic


Gear ratios

GT V6

1st 3.25:1 3.25:1

2nd 2.44:1 2.44:1

3rd 1.55.1 1.55.1

4th 1.00:1 1.00:1

5th 0.75:1 0.75:1

Final drive 3.31:1 3.31:1


http://www.allfordmustangs.com/artma...OVERVIEW.shtml

Hope this helps.

Mike

 

thats wicked i like it! makes a mustang dyno for automatic cars mroe efficient!

 

With the overdrive off, the car will also shift differently than it will with the o/d on. If you want better ET's, leave it on when you are racing.

 

"Drive" has been 1:1 on every auto I've ever met. Inside the transmission that's the most mechanically efficient ratio because the planetaries are locked up and there's no gear friction, which can otherwise eat several percent of the power coming off the crank. I'd like someone to explain why the top gear isn't routinely set up 1:1 like most trannys were decades ago--we're going to pay gear losses in the rear end anyhow, so why not set the rear end ratio to allow 1:1 in the transmission for the most-used gear? I hope there's a better reason than simple marketing (a fair number of people associate "overdrive" with "fast", but they've got the cause and effect backwards). Hopefully somebody can clue me in on this little mystery.

But back to the core topic, this thread has so far missed what I consider the most important reason to turn O/D off: engine braking. With O/D on and the selector in D, the torque converter will unlock and let the engine spool down (mostly) when you're off the throttle, almost like putting it in neutral (or pressing the clutch on a manual). With O/D off or the selector in 3, 2 or 1, that doesn't happen and you get engine braking very much like you'd get with a manual. That's useful on the downhill side of the mountain, so you don't have to ride the brakes for miles to maintain a sane downhill speed, and as I recall it's also handy in snow (which I've not had to drive in since 1988).