M-21 Overdrive Transmission???

[Ron Ramsey]

Is it possible to change the 4th gear in a Muncie M-21 4-speed manual transmission and make it an overdrive gear? My '66 has a 427/390hp with a 3.70 rear end. I know I can change to a 3.08 rear end ratio but that kills the "around town" performance. I want the best of both worlds, strong acceleration 1st through 3rd and then something reasonable on the Interstate at 70 mph (right now I turn almost 3500rpm at 70 mph). NCRS #33257

[John H.]

Re: M-21 Overdrive Transmission???

Ron -


There is no 4th "GEAR" in a Muncie or T-10; 4th is the direct ratio, and is accomplished by the 1-2 synchronizer locking the input shaft to the output shaft; only 1st, 2nd, and 3rd (plus reverse) have actual "gears". Only way to get an overdrive ratio (that will live) is to use a transmission that provides it (like a Richmond 5-speed).


John

[Ron Ramsey]

Re: M-21 Overdrive Transmission???

John,


Thanks for the info...It sounded like a good idea!

[Wayne M.]

Re: M-21 Overdrive Transmission???

Ron -- You could perform two changes, but it will cost. Trade your M21 for a wide ratio M20, and change your rear end to a 3.36. This will give you almost exactly the same performance in the first 3 gears that you now have with the close ratio and the 3.70's. Or, if you could live with a little less pep in 1 - 3, choose the 3.08's.

[Joe L.]

Re: M-21 Overdrive Transmission???

Wayne and John----


Absolutely correct. You know, over the years, the term "close-ratio 4 speed" has come to denote high performance and, in some folks' mind 'status'. The fact is that the wide ratio M-20 is a far better transmission for street operation, particularly for small block engines. It provides good 1st gear "scat" while allowing one to use a reasonable rear axle ratio for quiet, fuel efficient highway cruising. In fact, years ago when I bought the pieces to build my M-22, I only regretted the fact that the M-22 was not available in wide ratio. If the parts had been available in both ratios, I wouldn't have had to think for a second regarding which ones to buy.