# Rear gear change 3.73to ?



## bobby326 (Dec 13, 2009)

So after all the headaches and guess work the engine and tranny is all back together and running good. The car is a 65 tempest with a 67 326 comp262 cam, comp lifters, pushrods and magnum 1.5 rockers. Mildly worked stock 140 heads, light port work, stiffer springs and 7/16 screw in studs and m21 tranny. I currently have 3.73 gears in the 10 bolt but i drive bout 40 miles round trip to work and dont to to be screaming at 3,000 rpm's only going 55mph. This will be my everyday driver once the weather gets alittle better. Im on Long Island and we just got 28 inches of snow. I have anothe rear from my when i bought my 67 but it has 2.56's in it. My question is if i put the 256 rear in will it be a total dog with the cam i have. Im looking for a rear with 3.08's that i think will b perfect. Any opinions on rear ratio appritiated. Thanks


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## RunninLeMans (Apr 3, 2014)

Bobby - my '64 is very similar to yours, a moderate 326 and I went with the 3.08 Eaton posi. Very happy with it so far, with the 2.56 one-leg I could barely drive it. I recall is hums about 2,900 rpm at 70 mph with 25" tires.


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## bobby326 (Dec 13, 2009)

2900 at 70 with the 3.08. I could live with that. I have 15" torque thrusts and i think 26" tires. That night drop the rpms a little. Thanks


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## geeteeohguy (Feb 2, 2008)

With your M-21 close ratio, any gear less than 3.73 will be doggy off the line due to the tall first gear of the M-21. Originally, they only came on 3.90-up cars for that reason. I would recommend an M-20 with 3.23 or 3.08 gears. If you want to keep the M-21, even a 3.36 would be fine for what you are doing. I am running an m-20 with 3.36 gears on my '65 with 28" rear tires and it's a bit under 3,000 rpm at 70mph...about the equivalent of a 3.23 with stock (26.5") tires. No way in the world will a 2.56 work with a Muncie 4 speed of any kind. Can be done (BTDT!) but not recommended!


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## SANDU002 (Oct 13, 2004)

I am running 3:36 gears with my M-21 in my '65 and still get 13-14 mpg on the open road. 428 motor with mild cam and dual quad 580's.


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## azmusclecar (Sep 26, 2007)

Here's a link to use with different combinations with tire size calculator, rear gear ratio, rpm at whatever speed........

Calculators

As previously posted........a frined of mine who is an engineer AND mechanic told me GM uses a factor number of 9 to match transmissions to rear gear ratios
Example:
A munice CLOSE RATO has a 2.20 first gear ratio
If you pair it with say a 3.08 rear gear
You multimply 2.20 X 3.08 and it equals 6.76
That combination does not meet GMs SUGGESTED criteria of 9
Now,
A muncie Wide Ratio has a 2.52 first gear ratio
Stay with the 3.08 gear
Multiply 2.52 X 3.08 and it equals 7.76
This combo still doesn't meet GMs suggested criteria of 9

Now try a different rear gear say 3.36
Wide ratio 2.52 X 3.36 = 8.46
closer to 9 but not quite 

Gm's engineering goal was to just use that number as a basis for developing gear ratios in transmissions and offering different rear end ratios.

My OHC6 66 has a 3.08 rear gear
GM matched it with a 3.11 first gear ratio
3.08 X 3.11 = 9.57
Due to the engine being a 6 cylinder it needed a high 1st gear ratio to get it moving.
BUT.......you can see how I can keep a 3.08 rear, and use a Saginaw 4 speed 3.11 and enjoy both the driveability of it as well as it being able to pull off a hill without a lot of slippage or clutch chatter.

There was a reason GM offered the CLOSE RATIOs with stock 3.70 gears or higher.
2.20 X 3.70 = 8.14
A nice torqy V8 can make due with a high 7 or 8 and still drive okay.

GM also used the GVW to help get a larger mass moving by installing larger V8s

The number 9 is just a benchmark I use to try to determine a nice combination when matching trannys and rear gears. I use this when matching stock small block engines since some small blocks put out plenty of torque like a 400 small block Chevy.

I'm just giving you food for thought, how you drive your car and how good your clutch is and what size wheels and tires you are running. Larger wheel and tire knocks the actual rear gear ratio down.

Try the calculator and see, try lots of different combinations and especially watch rpms and so forth. It can give you quite an idea of just how much one change can make.

I have other internet tools if you need them just PM me.........


Transmission Gear Ratios

Helpful for race car, hot rod, and custom car builders.

More tech info
Transmission	First	Second	Third	Fourth
Chrysler A-230	2.55	1.48	1.00	--
Chrysler A-230	3.08	1.70	1.00	--
Chrysler New Process	3.09	1.92	1.40	1.00
Chrysler New Process	2.66	1.91	1.39	1.00
Chrysler New Process	2.65	1.93	1.39	1.00
Chrysler New Process	2.47	1.77	1.34	1.00
Ford Toploader	2.42	1.61	1.00	--
Ford Toploader	2.99	1.75	1.00	--
Ford Toploader	2.71	2.04	1.51	1.00
Ford Toploader	3.16	2.22	1.41	1.00
Ford Heavy Duty	2.32	1.69	1.29	1.00
Ford Heavy Duty	2.87	1.93	1.36	1.00
GM Muncie	2.20	1.64	1.28	1.00
GM Muncie	2.52	1.88	1.46	1.00
GM Saginaw	2.54	1.50	1.00	--
GM Saginaw	2.84	1.68	1.00	--
GM Saginaw	3.11	2.20	1.47	1.00
GM Saginaw	2.85	2.02	1.35	1.00
Warner T-16	2.41	1.57	1.00	--
Warner T-16	2.86	1.71	1.00	--
Warner T-10	2.54	1.92	1.51	1.00
Warner T-10	2.10	1.64	1.31	1.00
Warner T-10	2.54	1.89	1.51	1.00
Warner T-10	2.36	1.78	1.41	1.00
Warner T-10	2.36	1.76	1.41	1.00
Warner T-10	2.73	2.04	1.50	1.00
Warner T-10	2.36	1.62	1.20	1.00
B-W Super T-10	2.64	1.61	1.23	1.00
B-W Super T-10	2.64	1.75	1.33	1.00
B-W Super T-10	2.88	1.74	1.33	1.00
B-W Super T-10	3.44	2.28	1.46	1.00
B-W Super T-10	2.23	1.77	1.35	1.00
B-W Super T-10	2.43	1.61	1.23	1.00
B-W Super T-10	2.43	1.76	1.47	1.00
B-W Super T-10	2.64	2.10	1.60	1.00
B-W Super T-10	2.64	2.10	1.46	1.00


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## azmusclecar (Sep 26, 2007)

Oh, this math goes out the window when using automatics due to torque converters. They can be changed to meet the needs of the GVW and the rpms needed to provide that given torque.


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## azmusclecar (Sep 26, 2007)

I have a 64 Corvette with 4.10s and close ratio and I wanted to go to a more highway friendly gear. Say 3.36
2.20 X 3.36 = 7.39
I just wanted to do a rear gear change but I'm going to need to go to a WIDE RATIO
2.52 X 3.36 = 8.46
The Corvette only weighs 3200 and I have a nice 327/350 hp engine. All I wanted to do was an easy rear gear ratio change out but my Vette will lose much of it's low end drivability. Is it doable? YES but will I like it?????????????

So the change requires:
1. Rear gear ratio to say 3.36 and Trans change to wide ratio Muncie
or 
2. Change rear gear to 3.08 and Buy an M22Z with a 2.98 first gear ratio now being made aftermarket
OR
3. Keep the 4.10s and go to an overdrive tranmission.

What WAS just a low budget change has now turned in to a major decision and increased costs.
Good luck with your decision, I'm still on the ropes trying to decide............


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## azmusclecar (Sep 26, 2007)

SANDU002 said:


> I am running 3:36 gears with my M-21 in my '65 and still get 13-14 mpg on the open road. 428 motor with mild cam and dual quad 580's.


Light car with big block torque will always be a sweet ride using an M21..............:thumbsup:

Torque is a big deciding factor when picking a transmission combination.:smile2:


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## Bensjammin66 (May 12, 2009)

Could always sell the 4 speed and get a 5 speed. They have them using the same cross members, bell housings, linkage and all these days. Different clutch though and you usually have to hack the floor a tad. I have 373s with a 26" tall tire and 70mph is just under 2100 rpm with a .64 5th. Kind of pricey but OD makes a world of difference. With the money saved by leaving 373s ( assuming youre not setting up the diff yourself ) and selling the M21 you wouldnt be more than about $1200 off the mark of an OD trans i bet.


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## PontiacJim (Dec 29, 2012)

bobby326 said:


> So after all the headaches and guess work the engine and tranny is all back together and running good. The car is a 65 tempest with a 67 326 comp262 cam, comp lifters, pushrods and magnum 1.5 rockers. Mildly worked stock 140 heads, light port work, stiffer springs and 7/16 screw in studs and m21 tranny. I currently have 3.73 gears in the 10 bolt but i drive bout 40 miles round trip to work and dont to to be screaming at 3,000 rpm's only going 55mph. This will be my everyday driver once the weather gets alittle better. Im on Long Island and we just got 28 inches of snow. I have anothe rear from my when i bought my 67 but it has 2.56's in it. My question is if i put the 256 rear in will it be a total dog with the cam i have. Im looking for a rear with 3.08's that i think will b perfect. Any opinions on rear ratio appritiated. Thanks


So what was your oil problem with the engine??????


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