I have a 69 Camaro. I have a 383 sbc that does not rev high... maybe to 5500 or 5800. It also has a close ratio m-21 transmission. Currently the rear is a one legger with a 2.73 ratio. It used to be a 6 cylinder car... but I digress. I read somewhere that a close ratio transmission needs taller rear gears like a 3.73. I was wondering what a good choice for gears would be. I want that blast off feeling but I want to drive the car all around and not have the thing screaming in my ear at 55mph. My buddy says run 4.11s? I was wondering if I could get away with a set of 3.55's and still have fun with it and also have an all around drivable car? Thanks guys and gals I thought the urinal was funny its not relevant to the post LMAO!!!

 

I had both an M21 and M20 with a 12 bolt posi 355 rear. The M21 was the first trans, and I removed it because the tall first gear was a little to difficult to drive off of; bogging and bucking etc. The M20 is smooth off the stop. M21 or M20 have the same top gear, I believe. So RPMs should be the same. Off the top of my head I am going to say 3300 at that speed.

 

I had a 3.55 rear that was 3200 or 3300 at highway speed. All muncies are 1/1 in 4th. M21 does not have a tall rpm drop between gears but does have a taller 1st gear- 2.20 than M20- 2.52. That may be what you read about needing lower gears for an M21.

 

3:55 or 3:73 would be my choice
my 68 has a 355/m20 and 3:31

 

Sik, I was gonna say 3.55 before finishing your read. I have 4.10's and it's screaming down the highway, but I don't spend much time highway driving. Who knows, maybe I would with higher gears. If you plan on doing a portion of 55 mph+ driving, you won't like the 4.10's, and 3.73's would be marginal. The stroker motor makes gobs of bottom end torque, so low gears aren't as necessary as if you had a 350.

 

3.55 with 26" tire =
2752.6 rpm @ 60mph
2892 rpm with 3.73 gears and
3187 rpm with 4.11 gears.

 

Hey thanks for the responses guys... It sounds like 4.11s may be a little to extreme for the street and what I want to do with it. I guess it would be between 3.73s and 3.55s...

Buckmaster what do you mean when you had the m21 paired with 3.55's it would buck and bog? Meaning the car couldn't get out of the hole well, it felt sluggish in first gear? or it was too touchy and had the opposite effect and was not sluggish at all?

I can definitely notice the car currently feels piggish out of the hole with the 2.73s it will still burn tire but the car doesn't get up and go at first? It feels like there is a miss match in gearing... Which I believe there is!

 

I agree with those that chimed in on 55's or 73's. I've had 08's, 31's, 55's, 73's and 4:10's over the last 25 years. For a daily/highway driver which on occasion might be called on for a good hole shot I would say the 55's are best as a good compromise gear.

Taller gears like 73's or 4:10-11 are great for street racing, but I couldn't stand them for the freeway or long trips. I've had 55's for the last 10 years which give me enough grunt light to light and enjoyable driving on the freeway.

Good luck whatever you decide

 

I have the Gear Vendors OD that gives a .78 reduction.
3187 rpm's at 60 mph is reduced to 2479 rpm's.
2892 rpm's at 60 mph is reduced to 2256 rpm's.
2756 rpm's at 60 mph is reduced to 2150 rpm's.

 

I had 4:11"s, way too much gear for what the car is used for. Sometimes commuting 20+ miles town to town, I put in 3:25 locker(ford 9 inch). Works way better, and still lights up 275's with 8" rallys with a BBC. Every car and circumstances of how its used is a varible, just have to find whats good for you.