I've been reading all the different opinions on this,let me make sure i have this right. I'm running a small block with 26" tires,the best combination for street driven occasionally highway driving would be a m-20 with a 3.55 gear,But if I happen to go with a m-21 I should use the 3.73. Does this seem to be correct?

I have the m21 with 373's - Its fine for around town, but not so good on the highway. My next big ticket item was to be an overdrive tremec tko....unfortunately a blown up engine delayed my plans.

To get the same first gear multiplication as the M20 (2.56) x 3.55 you would need a 4.10 rear gear with the M21 (2.2), but then your mileage and highway RPM would really suffer. That is why the M20 is popular for a street driven car.

The RPM drops between shifts from first to second and second to third would be the same with either of the 2.54X3.55 or 2.2X4.10 combinations, but the drop from third to fourth would be greater with the M20.
Bruce

the M20 (2.56) x 3.55... 2.54X3.55

which is it, 2.56 or 2.54????

Or how about the more common 2.52:1 M20?

I think the 2.56 is only for the early versions (pre-66?)...

Just razzin' you Bruce...:p


I do agree with Bruce


To add,
M20 with 3.08 = M21 with 3.55s (not exact but close)

M20 with 3.31 = M21 with 3.73s (not exact, but close)

As Bruce mentioned, this would be in 1st, 2nd and 3rd gears... 4th is 1:1 in both trannys...

Hey Tony! Yes, I'm losing it. I couldn't remember which it was, so I went to the Auto Gear site and saw that the M22W gearset they sell is 2.56 compared to the 2.199 M22 and decided to use that. I forgot to change the second reference in my post, though. I knew someone would pick on me for my choice :p , so I did some checking and still messed it up.:clonk:

Oh well.:D

Bruce

I've been reading all the different opinions on this,let me make sure i have this right. I'm running a small block with 26" tires,the best combination for street driven occasionally highway driving would be a m-20 with a 3.55 gear,But if I happen to go with a m-21 I should use the 3.73. Does this seem to be correct?

Why would you choose a different rear gear ratio based on the 1st gear ratio? I would choose a rear gear ratio based on the gear I spend the most time in, like 4th, followed by 3rd.

Why would you choose a different rear gear ratio based on the 1st gear ratio? I would choose a rear gear ratio based on the gear I spend the most time in, like 4th, followed by 3rd.
The reason would be launching the car. I had a sb L48 with an M21 and a 3.31 rear gear and it was fine for day to day driving, but if I wanted to leave the line hard it would bog or spin the tires. A little deeper first gear would have made a big difference.

How would an M-22 with 3.73 rear perform. That's what I'll be running in mine but didn't give it any pre-thought.

How would an M-22 with 3.73 rear perform. That's what I'll be running in mine but didn't give it any pre-thought.

I think the M21 or M22 (same first gear ratio) should work fine with a 3.73. That was a standard configuration available from the factory. The M20 normally came with a 3.31 rear gear.

I mentioned the 4.10 in the earlier post because I thought the original question implied similar performance with the suggested combinations. In the end, the choice is determined by preferences and usage. A city street driven car or stop light drag racer might prefer the wide ratio if also driven on the highway, but a drag racer who will be shifting to 4th gear, or a road racer who will be shifting between 3rd and 4th might prefer the close ratio. Your choice may also depend on how much torque your engine puts out. A torquey big block could get by with a lower numerical gear, whereas a smallblock, especially a 327 or smaller, might need a higher numerical gearing.

I happen to have a M20 and a M21 that I am going to try with a 3.55, and use the one that I like best. I'll be using mine for cruising.
Bruce

I'd rather have an BW-ST10 with a 2.88 1st and 3.42's

Agree with mr 4 spd. A 2.88 low ST-10 is a real good setup for just about any combo.

The reason would be launching the car. I had a sb L48 with an M21 and a 3.31 rear gear and it was fine for day to day driving, but if I wanted to leave the line hard it would bog or spin the tires. A little deeper first gear would have made a big difference.


So you should select your first gear based on the rear end ratio, rather than the rear end ratio based on the first gear. We use an x-ratio ST-10 in our road racer for exactly this purpose. It has the top 3 gears similar to the M21/M22 but has a 2.64 1st gear for getting away grom the green flag quickly.

What type of gearing would you use with a saginaw with a 3.50 first gear setting behind a mild 408 (30 over 402) BBC? I was thinking of using a 3.08 rear gear since this is not a drag strip car. It is going to be more of a stop light to stop light with a little interstate travel. I figure the High gear 1.00 with a 3.08 should allow me some easy interstate rpms while the 3.50 first will give me a healthy off the line!

What type of gearing would you use with a saginaw with a 3.50 first gear setting behind a mild 408 (30 over 402) BBC? I was thinking of using a 3.08 rear gear since this is not a drag strip car. It is going to be more of a stop light to stop light with a little interstate travel. I figure the High gear 1.00 with a 3.08 should allow me some easy interstate rpms while the 3.50 first will give me a healthy off the line!

3.08 is the perfect rear gear with a 3.50 1st
But a Saginaw isn't likely to appreciate the torque of a BBC for very long!

3.42 low ST-10s work real well with 3.08s. They are not something that I would sidestep the clutch at 7000rpm with but for just about everyone else it will hold up fine.

I plan on running the saginaw for a year or two then step up to a 6-speed TKO and switch to 4.11's! Work with what I have then once the car is on the road it's play time!

Why would you choose a different rear gear ratio based on the 1st gear ratio? I would choose a rear gear ratio based on the gear I spend the most time in, like 4th, followed by 3rd.

My reason is I have been looking at both m-21 and m-20's to replace my saginaw,I have found that I will not be able to be too picky on which trans I get based on my budget,I used a rpm calculator,and determined there would only be a 153 rpm difference bettween the 2 combos @65mph,and all though I would prefer the m-20/3.55 setup,I wanted to make sure m-21/3.73 would launch comparably if that in fact is what I wind up with