Im currently restoring my 70 chevelle. The 402 is about out of the machine shop. its a 402 20 over 9.5-1 comp very mild head work. Edelbrock RPM air gap, 750 holley. gear drive, hei with a afermarket coil. not sure on cam yet, probably a 280 duration solid. you get the point. Im runnin it through a M -21 and ive been looking for a posi, found a 12 bolt 2.73 posi in the paper. Just curious to how my combo may work. Anyone with a similar set up? i know the M-21 is a closed ratio, how will that translate with the 2.73's

What kind of gears are you running now? Close ratio Muncie and a 2.73 rear is not a particularly good combination in a heavy Chevelle if you want to have good bottom end power.

It won't work. If you want the 2.73s get a 3.42 low ST-10.

well right now it has the stock 10 bolt, think its a 3:08, just figured the 12 bolt would end up with different gears, just want the 2 leger for now. what gear ratio would you recomend. it will see highway miles

3.55's

i have that rearend in my 66 with an m-20 .......it will work, i had the 3.55's in my last car with a 454 and m-21 which i commuted 36 miles each way to work every day.....i wore the motor out in a year. most people that go with the 355 lower gears don't drive their cars every day so they don,t have to live with it. yeah.....my car is no stoplight screamer but in reality...it is really unsafe to drive that way since stoplights are usaully populated areas, and these gears really are fun when you're runnin down the highway and some one wants to "play"....i'll never forget the time in my last car when (with 355's) playin with a dodge stealth on interstate 5 in cent ca. i was all smiles until i topped aout and he still had one more gear!!!!!....the hummiliation!!!

Originally posted by SS66rag:
i have that rearend in my 66 with an m-20 .......it will work, An M20 and an M21 have different gear ratios. The M21 is a close ratio trans, the M20 is a wide ratio. Use 2.73's with an M21 and you are asking for problems.

I think this would be a poor choice...

For such "high" gears, I'd use an M20, or a Richmond 5-speed...

M20 (wide Ratio) has a lower first gear (and second and third gears) than the M21 (close ratio)... All Muncie 4-speeds have the 1:1 4th gear.

You will really need the lower first gear in those trannys to get the car going...

Sure, it will work, but you will wear the clutch out much faster because you will have to slip it much more when taking off... And low speed accelaration will be bad until the rpms get up to wear the engine makes good power...

I currently run 2.73/posi with a Richmond (3.04 first gear), behind my 396... Its okay, but I'd like to go a little lower in first gear (3.27, next available for the Richmond)...

Now how sure are you that its really an M21?


Some quick math (assuming a 27-inch tall tire)... 750 RPM at 10 MPH in first gear... You car probably idles above that, meaning you would have to slip the clutch alot...

4500 RPM at 60MPH, in FIRST Gear!!!

Originally posted by 1966_L78:
Some quick math (assuming a 27-inch tall tire)... 750 RPM at 10 MPH in first gear... You car probably idles above that, meaning you would have to slip the clutch alot...

4500 RPM at 60MPH, in FIRST Gear!!! where do you get that formula? I've been trying to figure out what my gear ratios would be with my 308 and 26" tire and certain RPMs and MPH. I don't mean to steal the topic but for future referance Please
Zac graemlins/thumbsup.gif

wow posted this question some time ago, still gettin replies. I didnt really think the M-21 and 2.73's would work out great, just didnt now how bad it would be. Im desperatly looking for a reasonably priced used posi. came across one for 200 that had the 2.73 grears in it, thus the question was posed. Still havent bought a rear yet.... hope to find one at the Wichita swapmeet in Feb.

If its a 12 bolt for $200,grab it..even w/2.73's its worth it.I get $200 for a 12 bolt non posi,and I can't keep enough of them.

Anthony, I run a 355,M21,342 gears.Seems to work very good for street and a little fun. Depends on what you really want to do.I like the tight turns on the old paved backroads with a little down shifting works smooth not to man rpms between gears on down shift :D Ralph

The 2.73 is an excellect choice for the hiway, but an M-20 would be much more compatable with the 2.73, rather than an M-21.

Young Gun 71
Here is a site that will give you the " gears " you need.
It lists every tranny, gear, rear tire height, RPM per gear etc etc. etc etc.

graemlins/hurray.gif One of the best I have ever seen.

www.dalesplace.com/htm/information/rpm_calculator.htm (http://www.dalesplace.com/htm/information/rpm_calculator.htm)

young gun 71:

I kind of just figure it out in my head (and some scratch paper)... I have used a spreadsheet before, but I have switched computers and never remade one...

Basically, if you know the tire size or height, you can calc the diameter, make sure to convert that to feet...

ex: 27 inch tire-> 27 in * PI = 27 * 3.1415927= 84.82 inches = 7.069 Feet

1mile=5280 feet, so divide the 5280 by the tire circumference, that will give you axle/wheel revolutions per mile...
5280/7.069 = 747 Revs per mile

At 60 MPH = 1 mile per minute, so the axle revs per mile is also the axle RPM at 60 MPH...
747 Revs per mile = 747 RPM (wheel)

Multiply the axle RPM times the gear ratio, and that will give the driveshaft RPM at 60 MPH...
747 RPM*2.73 (rearend gear ratio) = 2039 RPM (driveshaft)

Multiply the driveshaft RPM times the transmission ratio (whichever gear you decide), and that will give engine RPM at 60 MPH...
2.20(M21 first gear) *2039 RPM = 4486 RPM at 60 MPH, in first gear with an M21, or approximately 4500 RPM

You can divide that RPM by 60 will yield RPM/MPH, then just multiply by the speed you are interested, and it will give you engine RPM...
4486RPM/ 60MPH = 74.8 RPM/MPH
74.8 RPM/MPH * 10 MPH = 748 RPM at 10 MPH

Pretty simple...

Create a spreadsheet to make comparisons easy...

A friend of mine had a 67 camaro with 3.08 gears and a wide ratio muncie, we put a mild smallblock 400 in it and it worked fine. You could actually take off from a dead stop in third gear with this combo because of the torque the 400 made. I think you'll be fine with your big block, 2.73 and close ratio combo if this thing isn't being built to win races---hell alot of cars came with powerglides and 3.08 gears. I'd try the 2.73's, you can always re-gear later. Bob

Ditto baddbob71!!

I ran my First Gen. with a 406 SB, an M-20, 3.31's and 26.5" tall tires for quite a while before I quit driving it years ago when my clutch was not working right!!

Just backed her out of my parking spot using reverse, killed her and started her in third gear and kept on chugging!! NOT the best idea but got me several months of commuting before I bought my '87 Chevy Sprint ER 3-banger, corn-popper car...

YES, it's amazing what grunt and a little clutch slippage does for moving a car!!!

pdq67

PS., I dropped everything off my car AND still don't know why it wouldn't shift b/c the clutch plate still has it's manufacturers machining marks on it and the P/P and F/W look fine?????? All that and the T/B was only 10,000 miles or so old???

I'm gonna put everything back on my big motor except for the F/W and see if it's OK now!