OD question

How many revs does your engine drop by when you engage your OD?

About 350-400 RPM's, on a '74.

Depends on the speed you're going. Often people think of an OD as "dropping your RPMs by about 500" with respect to common highway driving speeds. Do you want specific numbers for specific speeds? You should be able to calculate it just by knowing the gear ratio of an OD (I don't know it!).

The OD ratio is 0.802 on the 4-sync box, it'll drop your rpm by about 20%.

http://www.autochart.com/MGB_OD_Trans.html

http://www.autochart.com/MGB_Trans.html

Like these guys said it varies with speed. Here is an old post by John Weimer (edited by me) showing the relative drop at different speeds.

John D. Weimer Wrote:

4th gear rpm 3,072 overdrive 2,519 @ 55 mph Difference of 553 RPM
4th gear rpm 3,351 overdrive 2,748 @ 60 mph Difference of 603 RPM
4th gear rpm 3,631 overdrive 2,977 @ 65 mph Difference of 654 RPM
4th gear rpm 3,910 overdrive 3,206 @ 70 mph Difference of 704 RPM
4th gear rpm 4,469 overdrive 3,664 @ 80 mph Difference of 805 RPM

http://www.mgexperience.net/phorum/read.php?1,872039,873402#msg-873402

Thanks Guys for the great info. Exactly what I was looking for.