Need some help with trans selection for my 67 vert. Im putting a 406 small block in the car. I have a 12 bolt 3:31 going in as well. My overall tire height is 25 inches. Which trans would you guys run a M20 or a M21?? I have both. Thanks!!

Unless you have a bunch of torque down low and a heavy flywheel, you'll be happier with the 2.56/2.52 (depends on which M20) first gear vs. the 2.20 of the M21.

The 406 is built for more low end torque. Do you look at the gearing in the trans the same as a rear? Like 3:31 compared to 3:73 and 2:20 close compared to 2:52 wide.

multiply first gear x rear gear.
I shoot for 10 overall or greater
The m20 is by far the better choice for a 3.31 rear.
That's why I run a Richmond rs5,with a 3.31 gear,my first overall is close to 11 yet in top gear my rpm is decent with a close ratio spead similar to the m21/22
The Tremec 5's take it a step further with a low first and a overdriven top gear but with a spead similar to the m20

I ran some numbers on a simulator for the 406 and I'm going to be real close to 500hp at 6k and 500ft lbs of torque at 4k (roughly) I was thinking with these numbers the m21 would be more street friendly and easier for my wife to drive. Any thoughts?

I ran some numbers on a simulator for the 406 and I'm going to be real close to 500hp at 6k and 500ft lbs of torque at 4k (roughly) I was thinking with these numbers the m21 would be more street friendly and easier for my wife to drive. Any thoughts?
It would be easier to take off with a lower gear such as the m20's 2.56/2.52, the first gear wouldn't be all that quick of a shift, and if she let the clutch out too fast it probably wouldn't want to die as easily since theres less of a load on the motor.

If your 406 has a 8/71(that's torque)run the m21
If you change to a 4.88 gear set run the m21.:hurray:very fun !
If you desire a nice drivin car with your 406 and 3.31 with a ok first gear and decent highway manors-m20.

OK, Thanks guys! I will be refreshing the M20 to go in the car. Good to talk to you all again.

They aren't that hard to swap out. Try the M20. If you don't like it, take a couple of hours and stick the M21 in it.

I did a lot on research and I'm very happy with my combo.

ZZ383
Autogear M22W 2.54 1st gear
3.42 Truetrac.

Very easy to drive around town in 2nd gear all day. And has real long legs for high speed highway cruisin'. 4th gear is 2500 rpm @ 65mph with 255/60r15 BFG's.