3.55 posi w/m-20

[ken girven]

Hello all,

While my 69 350/350 hp 69 is in the shop getting restored I was thinking about how the car will run on the street (yes it WILL be driven). It has a 3.55 gear ratio and the m-20 transmission. I have never registered the car so I have never really driven it on the street except for a few illegal blasts up and down the road i live on . I was wondering if anyone has a similar gear ratio/tranny setup and how this combo works on the street in terms of acceleration, highway driving etc. By the way it has the stock blueprinted cam and is rebuilt to stock specs (carb , ehx. manifolds, etc)......as I wait patiently for my car to return.............

Kenny- #31871 (YES I am finally a member and reall enjoy the Driveline and The Restorer)

[Shannon Burgess]

Re: 3.55 posi w/m-20

Kenny,

I have a '69 350/350 with a 3.55 rear and M-20 also. Although I don't have anything to compare it against, I think the tranny/rear end combo is a great match for street driving. I turn around 3000 rpm cruising at 70 mph. I do think that the high revs at highway speeds would get a little annoying over long distances, but I don't do much in the way of long freeway trips.

I'd be glad to help with any other specifics you're interested in.

Shannon

[Joe L.]

Re: 3.55 posi w/m-20

Ken----

The 3.55:1 rear gears coupled with the M-20s 2.52:1 low gear is an excellent combination. In fact, it provides nearly the same first gear overall multiplication ratio as an M-21 with 4.11:1 rear gear. But, in high gear, you get the benefit of lower noise, engine wear reduction, and improved fuel economy. Of course, you do lose some performance as a result of the greater spread between gears, but most things involve some sort of trade-off.

As a matter of fact, I consider the M-20 to be a much better street transmission than the M-21. Everyone gets hung up on this "close ratio 4 speed" thing. A close ratio 4-speed is better for racing, but a wide ratio is better for the street. Which one is better for you depends on whether you do more racing or more street driving. It's as simple as that.

[Wayne M.]

Re: 3.55 posi w/m-20

Kenny; I will echo Shannon and Joe -- the wide ratio with 3.55 rear is a nice combo. As Joe says, it's like a 4.11 with the close ratio, except for when you shift into 4th gear. In fact, the "spread" between a 1-2 shift, 2-3 shift are identical for the close and wide ratio trannys. Another combo is the wide ratio with 3.08 rear -- it's very close to the close ratio with 3.55, except on the 3-4 shift.

..... Spread (Ratio)

.Shift 1-2 .. 2-3 .. 3-4

M-20 1.34 1.29 1.46

M-21 1.34 1.28 1.28

[Roger Coen]

Re: 3.55 posi w/m-20

I have the 3:55 with M-21. Winds kind of high on the freeway. Nice around town tho.