I want to use a manual 4speed behind my 235 six. What gear ratios should I be looking for. It will go in a passenger car. I am looking at some Saginaw transmissions. They have 1st gear ratios from 2.84 to 3.50 with different spreads for 2nd 3rd and 4th. Any suggestions? Thanks.

Depends on your rear axle ratio.

MBHD

What is the rear axle ratio going to be?


MBHD

Ratio will be about 3.55.

I would not go any lower than a 2.84 1st gear w/355's


MBHD

X2: good starting point is 1st gear × axle ratio = 10 or more, especially for a small engine with mixed use.
Easy math, using "calculator" function:
Enter the axle ratio (3.55, etc.).
Press "1/X".
Result × 10 is 1st gear ratio.
10 ÷ 3.55 = 2.82:1.
10 ÷ 3.23 = 3.10:1.
10 ÷ 3.73 = 2.68:1.
10 ÷ 4.11 = 2.43:1.

OK with less than 10:
big motor/light chassis
heavy flywheel
highway only
Needs more than 10:
small motor/heavy chassis
big cam
aluminum flywheel
traffic only

It used to be the thing to put the four speed case (ever so slightly modified for the shifting o/d rail) in front of a sag overdrive unit, and making it a poor boy's 5 speed. Armed with a manual o/d switch, a split manifold six could make a decent diesel truck sound (and about as much accelleration) by using all 10 available ratios. The o/d would open up even more rear axle choices- use it as an o/d, or as an operable gear. Finding the o/d unit will be tough although older borg warner o/ds may be adaptable. The 3.11 low gear four speeds were plentiful in Vegas.

It;s really only 8 ratios but still fun. It's done with the Saganaw 4 speed and the OD from a 3 speed saganaw of the same vintage. They used the same case. You have to use the main shaft from the 3 speed and sliders from the 4 speed. Lots of "how tos" on the net. The older Borg ODs wont bolt up. Saganaw 3 speed ODs are getting hard to find and the special solenoid used only in the Saganaw set up is even harder. NEVER throw away a broken OD solenoid. They can be fixed! It wouldn't help in a racing application because of strength and the time it takes the OD to shift but for fun on the street it is a fun combo. I'm going to put one behind the 4 cylinder in my roadster. I have often thought of using the plentiful Borg ODs for other applications as a stand alone. It would take modified front cover and a yoke.

Eight speeds?. Never let the facts get in the way of a good yarn. I recently sold a saginaw o/d three speed (broken) Back to the same dealer that I had purchased it from (in working condition) some 25 years prior for the same 75 bucks at a Fort Worth swap meet- when he sold it to me 25 years ago he said the same thing about the solenoid being hard to find. When he bought it back he said it was not the "hard to find" saginaw solenoid but an earlier one--hmmmm? Still, it worked fine for me and a good return on my money.

The $75 you paid 25 years ago was not what you got. 1st, 1stOD, 2nd, 2ndOD, 3rd, 3rdOD, 4th, & 4thOD. Thats 8! The solenoid used in the later Saganaw 3 speed OD has a longer shaft and the older GM, Stude etc solenoids will not engage the paul in the OD unit. The later Fords might work but I have never compared them. Here are some pics the traneys are mid to late '50s GM and mid 60s Saganaw. same case as 4 speed. The long shaft solenoid with the angled cap is the only one that works in the late Saganaw. But it's just a yarn that was printed in Hot Rod in May of 70 I believe. Lots about this on the net. Beater

I was countin' All of them, not just the forward speeds (poetic LIEcense). From what you say, and the fact that the solenoid was never switched, then it well may have been the correct one after all. Many 2.54 low four speeds became 2.20 lows when money got involved. I paid him 75 nice dollars for a nice tranny 25 years prior, and when he bought it back he paid 75 old rusty dollars for an old rusty tranny. 75 dollars more than the landfill would pay... Thats a nice return.

If you do the math, you'll find that (unlike a purpose-built truck auxiliary transmission) you have a theoretical total of 8 ratios, but some are so close together as to be useless.
Example, using .70 OD:
1st 2.84
1st OD 2.00
2nd 2.01
2nd OD 1.41
3rd 1.35
3rd OD .95
4th 1.00
4th OD .70
Look how close 1st OD is to 2nd, 2nd OD to 3rd, 3rd OD to 4th: they don't do anything.

Using a close-ratio box:
1st 2.56
1st OD 1.79
2nd 1.81
2nd OD 1.27
3rd 1.37
3rd OD .96
4th 1.00
4th OD .70

Still nothing...

looks like the first liar aint gotta chance! But I do get 1.988 for first gear o/d in the first scenario.

Some of those are pretty close for sure and not really of much use. With a close ratio 4 speed Saganaw the 4thOD is really all I,m interested in. In the 3 speeds 2ndOD is a handy ratio.

There are times when an intermediate gear is really nice - long slow upgrade where you need to keep the RPM in the right spot, but generally a ratio change of 15% or less isn't worth shifting (unless it's into top, of course).

I put a Vega 4-speed behind a 59 235 in a 50 Chev 2-door with a 56 3.55 rear and loved it. The 3.10 was made to save clutches and frustration. New T-10's with a 2.88 low gear are readily available from Summit etc.

The one car I wish I had back..........

Dang, I've been torturing myself over finding a 4 speed Sag to marry in my '69 3 speed + overdrive. All that torture is laid to rest with a beautiful numbers chart! Thank you. I really should have known better from all my triple chainwheel touring bicycle experience (lots of nearly identical crossover gearing). Thanks so much.

BTW, I'm a daily driver only vehicle. The '62 3 sp+ od is the one Jack Halton sold to me in '98 at Latimore, PA. I do split the gears and have 6 forward gears, 2nd overdrive is a fantastic gear for my '57 3.90 rear.

Anyoone consider running a DN 4+3 used in '83 to '86 (I think) corvette's ?

super T10 with a DN OD on the back electrically controlled.

3.06 1st.

For what it is worth, I found an old Patrick's catalog. When doing the Saginaw and 3:55 gear conversion he recommended using the 3:11 first version as the rpm drops between gears kept a 235 in the powerband. It did not mention if a hot rodded motor made a difference, but logically it should .....

the rpm drops between gears kept a 235 in the powerband

Not sure what that means. All transmissions are designed to do that, but with a specific engine.