rajin cajin

I have a 1972 corvette NOM L82 that had an auto transmission. I converted over to a M21 (close-ratio) from a 1973 donor car that had a 454. My car has a 3.08 rear differential currently. I have the option to pick up the diff from the donor car. I'm going to look at it tomorrow to determine the gear ratio. I suspect it's a 3.36/3.55/3.70 and not a 3.08. The question is with a small block L82 and M21, what is the ideal gear for the rear? My understanding is that the M21 needs 3.70 to 4.11 and 3.08 to 3.55 is more for the M20. Am I wasting my time putting in 3.36 or 3.55 in a small block / M21. Any info appreciated.

tpfarm

I have a '73 with M21 , 3.70 rear gear. What I can tell you is that at 70 mph she turns about 3300/3400 rpms. It doesn't like the freeway much.

tracdogg2

Standard ratio for a small block with the m21 is 3.70 gear. Big block is 3.55. Nearly all L82's with auto's were 3.55.
3.36 is not a good gear with the M21. 3.55 is 3200 rpm at 70.
Mike

rajin cajin

Thanks guys. Just the info I was looking for. Keep my fingers crossed for a 3.55 when I go look at it today.

c3_dk

I have L46 with M21 and 3.70 rear.
I've done a long trip with 70-80-90mph, I have no problem with 3.70/high rpm.

I would for sure go for 3.70, and that is what GM also tought was the best, that why they made it :- )

7T1vette

If you want to drag race the car, you need the 3.70 or steeper rear gear. If you want to drive comfortably and economically on the highway, the 3.08 is probably the best way to go. The 3.36 rear gear is a decent 'middle-ground' and will put you at 2800-3000 rpm at 70 mph.

Choice is yours.

rajin cajin

Its actually a nice ride on the interstate now. I just wanted a little more off the line seat of your pants feel. Maybe a big block is in order...

7T1vette

A 5 or 6-speed gearbox with a steeper first gear and overdrive in high gear would be just the ticket for what you want...best of both worlds without any sacrifice to either!!

If it were me, that's the route I would go rather than to mess with the diffy.