The Stovebolt Site Forums The Shop Area Engine Shop Will the 235 with the sm420 Pull the 3.08

I’m sorry if i ask this several times in deferent ways its just I cant finger out which way to go if I need to use the 3.40 or 3.08 rear i want it to go 60-70 but not be doggy in town. This is just a fix for the next few years until I do that distressful thing and put a 350 in it.

Help
Curt

3:40 will not be doggie in town, 3:08 will be more doggie in town. 3:40 will get less gas mileage on the interstate than the 3:08. If it were me put in the 3:08 to be ready for the 3 fitty. Great fun. Thanks couldn't resist, thanks.

Thanks 52pu Ill keep looking for the 3.08

Curt

The SM420 is kind of unique among four speeds in that it has some really wide spacing between gear ratios.
There is a fairly big leap between 3rd and 4th, for instance. If you set it up so that you can actually use first gear on the street and make a quick jump to second it won't be so bad, I suspect.

I made a excel chart of engine RPM versus speed that I can email you if you are interested. I don't have a web host set up so I can't post it, but maybe someone else can.

I have decided that I want 3.08 gears with 28" tires with my 261, giving me a nice cruising rpm of 1980 RPM at 55 mph, 70 mph at 2500 rpm, and the ability to use 3rd up to 60 mph.

A 3.55:1 will give 2900 rpm at 70 mph. 3.40 wouldn't be too bad, the differance might be about 250 rpm at 70 mph between the 3.08 and 3.40, which isn't going to break the engine.

The gear ratios of the SM420 are:

1st- 7.05:1
2nd- 3.55:1
3rd- 1.55:1
4th- 1:1

These are the gear ratios I have found:

(Think the SM-420 has a large gap between 3rd and fourth? Look at the 3rd-4th gap in RPM's on an SM-465!)

GM Truck, four speed standard transmissions. Built by Muncie.
Installed in: 1947 - 1967 Chevrolet and GMC trucks up to 2 1/2 ton.
Model: SM-420 - Bearing Retainer/Bell housing opening size - 4 11/16"
Gear Ratios
1 - 7.06

2 - 3.58

3 - 1.71

4 - Direct

R - 6.78

Oil Capacity
5 1/2 pints

Installed in: 1968 - 1991 Chevrolet and GMC trucks up to 2 1/2 ton.
Model: SM-465 - Bearing Retainer/Bell housing opening size - 5 1/8" Gear Ratios
1 - 6.55

2 - 3.58

3 - 1.57

4 - Direct

Whoops, I guess I shouldn't post ratios off the top of my head...

Thanks for the replys 4onthefloor if you email me that excel page i can post it on my page

Http://www.bsu.edu/web/clschleibaum/pickup.html