Hi all. I recently blew up my tranny (M-21). I got it replaced and a buddy of mine is going to help put it in hopefully today or tommorow. Anyway, I read somewhere that the first gear ratio multiplied by the rear end should be around 10:1. Is this true? I recently found that I have a 373 rear, and with a 2.46 first in the new M21, it would yeild 8.206:1. Would I benifit from doing a rear gear swap to get the 10:1 ratio? Would I notice a difference if I swapped to a 4.30, which would give me 10.406? Highway cruising is not much of a problem.

Thanks a lot,
Chris

I thought the M21 had a 2.20 first gear.

A 10:1 fimal ratio in first gear is great, but you also have 3 other gears to consider. How much time do you plan on spending in first gear?

I made a typo in my first post. It should have been 2.42. Everything that I read on line said that it has the 2.20 first, but the guy that rebuilt it for me said it had the 2.42. I think that I am going to double-check that fact. As far as the time in first, just normal driving, other than a few 0-5500rpm. I am really not worried about highway-cruiseability.
Chris

The overall ratio is 8.20 (2.2 x 3.73). A steeper first gear ratio will make the drivability much better. A M-20 has either a 2.52 or 2.54 first gear and Super T-10s are available with 2.43, 2.64 & 2.88 low gear ratios. An overall multiplication of around 10-1 should save clutches also. There never was a chevy ratio of 2.42 or 2.46 offered.

Hmm....Now I wonder if the guy charged me more to install the "2.42" first. Time to investigate. Thanks for the help.
Chris