I know the M-20 is for gears like a 3:07 but if I wanted to up the rear gear on my 69, what gear would you go to for a good street gear with a mild 454 ?
Thanks.

What are you looking to do? Race, fun street car with no highway, fun street car with limited highway... "Up" the rear gear??? I presume you mean larger numbers (3.73, 4.10 etc)...

The M20 generally was for gears up to 3.55... And generally 3.55, 3.73 etc got the M21...

the ratio difference between 1st and 2nd as well as 2nd and 3rd is about the same for either M20/M21/M22 (stock GM ratios)...

The "wide ratio" is only the 3rd/4th shift...

an M20 with 3.07 gears is very similar to an M21 with 3.55 gears (calcs to about 3.51)... M20/3.31 similar to M21/3.73 (calcs to about 3.79)...


A 454 with 3.31 or 3.42 will still usually preform pretty well (lots of torque to pull those gears)...

I ran my 396 with an M20 and 4.10s... It got old after a short time... swapped to 3.42, and it was nicer (I would have done 3.07 or 3.31 IF i had found them available)...

x2
Anything 3.42 and numerically lower, I would use a M20. 3.55 and numerically higher, I would use a M21 or M22.

I ran 3.55s for years, with a 28" tire. I'd go 3.42 or 3.55. Since I doubt you'd feel any difference in acceleration, might as well go 3.42.

3.42 seems to be a very popular gear. I ran 3.55 cause that's what came with the car. I have 4.11s right now and I used to have 4.56s. You don't want either for a street car with an M20.