what is the best tranny to run for best performance (quarter mile times) a M-20 or M-21?
Rear gears are 3.73 with a 27.1 inch tire p255-60r 15.

I would install the M21.

M20= 2.52:1 1st
M21= 2.20:1 1st

If you used an M21 with your 3.73:1 and then installed an M20, your vehicle would feel like you had a take off feel of a 4.27:1 differential in 1st gear.

To compare the close and wide ratio transmission gearsets here is a ratio formula:

M20 1st gear ratio x differential ratio / M21 1st gear ratio = 1st gear take off feel.

2.52 x 3.73 / 2.20 = 4.27:1

The lower geared M20 sounds good on paper but, it is was not intended for repeated abuse. Drag racing will surely destroy most M20's.

You would be better with an M22 (2.20:1 1st) or an M22W (2.56:1 1st) gearset for repeated abuse. Either of these gearsets will install in your existing Muncie as long as you have a 1" case. But, please keep in mind that an M22 is no guarantee against breakage.

jody, I only go to the track about 3 or 4 times a year so I am not a hard core racer. But when I go Iwant to get out of the whole and do the best I can so that what I was asking about not for an every weekend drag car. I read some of your post and see you sell these transmissions is there a way I can call you to talk about a poss. purchase.

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