Looking For Info on 3-Speed Trans Ratios

[C10 - C90 Bill]

I was under my '72 C-10 today and wrote down the numbers off my 3-Speed trans. The number located on the main case is 3925847. The trans seems to be original to the truck as the date code reads Feb 20, 1972 (B202).

I'm trying to find out what gear ratios the trans has, especially 1st gear.

Any info would be appreciated.

[kyull67]

oops.

[WorkinLonghorn]

The standard 3-speed is; 1st= 2.85-1
2nd=1.68-1
Rev=2.95-1
The HD option 3 spd = 1st=3.03-1
2nd=1.75-1
Rev=3.02-1
I don't know which you have but the HD came standard with 350 and BB motors I believe.-cougar

[Brad]

Code H is 3 spd HD Muncie, code K is 3 spd McKinnon, M is 3spd Muncie, and S is Saginaw. There were 2 Saginaws: CS models got the synchromesh with ratios 2.94, 1.68, 1.00, 3.14 rev. CE models got the fully synchronized with ratios 2.85, 1.68, 1.00, 2.95 rev.

[C10 - C90 Bill]

Appreciate all the replies.

Just to make it clear, it is a manual trans.

I believe it is not the Heavy Duty type. The truck is a Custom C-10 with a 307 engine and has a synchro first gear.

Why I am asking is, it is a strange set-up. I have what I believe are 3:73 rear gears. When I first got it, I could not go over 60 MPH without it feeling like it was revving too much. I put on 265/75/15 tires on the back and gained about another 5 MPH which made a big difference. But first gear felt too high, even before the tire change. After the tire change, of course, it got a little worse. If I were to guess, it feels like I got a 2.00 or 2:05 first.

Did GM make one with high first gear ratio, like it seems I have?

[Brad]

You probably have a 2.85 1st since it's a V8. I'm not sure I understand what you're asking.

[C10 - C90 Bill]

Brad, It sounds like I'm supposed to have a 2.85 first, but it don't seem like it. First gear feels too high. It's almost like I'm starting off in 2nd gear.

[Brad]

Are you sure you have an original rearend? Could it be that it's been swapped out for a Camaro rearend giving you say, a 2.73 rear gear ratio? To check what ratio you have jack up the rearend and make a mark on the tire and on the driveshaft. Spin the tire 1 revolution and count how many turns it spins the driveshaft. If it's 3 & 3/4 turns its a 3.73, just over 4 turns and it's a 4.10

[C10 - C90 Bill]

I've never proved what ratio I have, but I feel pretty certain about it being a 3.73. Check back to my second post, if you would, and you can read about my over-revving on the highway problem.

If GM didn't offer a trans with a much higher first than a 2.85, then it's almost like someone along the line replaced first gear with a much different ratio. Which, I guess is possible. I'll probably just try another trans one day.

[WorkinLonghorn]

Are you used to driving some other vehicle with a four speed? A T-10 truck 4spd has a 3.44 1st.My truck has a N.P.435CR which has a 4.56 1st.And of course the granny low on a 465 is even lower (6.55).But even 2nd in a 465 is still much lower than your 2.85 at 3.58.
If you have the standard 2.85 1st and 3.73 rear and 31" tires,you should be able to hit 43 MPH at 5000 RPM in 1st.-cougar

[C10 - C90 Bill]

Cougar, You just about read my mind. I was about to say that myself. I am pretty much used to granny gear 4-Speed and 5-Speed trucks and 10-Speed Road Ranger transmissions.

But,

The half ton pick up I drove daily just before this one was a '66 C-10 which I believe also has a 3.73 rear. Starting off in that truck seemed more normal to me. When I jumped over to the '72 C-10, I could tell a difference. What I need to do next is find out if the '66 trans has a lower first. As you probably know, they are the old style Muncie non-synchro first trannys. Typical of those not holding up real good, I went thru 3 of those trans in the 9 years I had that truck on the road. The last one I put in, was out of a '61 Bel-Air car with a 235 6 cylinder. All 3 trans I used all seem to have the same ratios.

What I need to do next is to find out what those old 3-Speeds had for a first gear ratio and that could possibly answer my question.

[WorkinLonghorn]

Well,unfortunately I only have data for 1969 from the original GM "Truck Data Book" from that year and they only list the two 3-speeds I listed in my first post;regular (2.85-1) and HD (3.03-1).There was also an overdrive unit available on the C-10 with 4.11 rear only,but it has the same 2.85 1st gear.
These two 3-spds were fully sync'd.-cougar

[Brad]

[WorkinLonghorn]

Actually Bill,there is also a 3-speed listed for the El Caminos that had the 350 or 396 engines (in 1969 anyway) that had a 2.42 1st gear,1.58 2nd,and 2.41 Rev.
Is it possible somebody had one put in a pick-up?-cougar
Just noticed that it was a floor shift though.

[WorkinLonghorn]

What I need to do in time is to find one of those HD 3-Speed trannys with the 3.03 first gear.

There should be lots around.Here's what the 1969 GM "Truck Data Book" says: "A special heavy-duty transmission is included when either the 350 V8 or 396 V8 engines are ordered..." "This unit is also standard on all 4-wheel drive (KA 10-20) engine applications."-cougar

[C10 - C90 Bill]

Cougar, It sure seems like I got a 2.42. I'd like to prove it one way or another and I'm wondering if the numbers I posted in my first post would tell us anything.

Also wondering how to tell an HD 3-Speed compared to a Standard one.

I can find plenty of info on 4-Speeds, but the 3-Speed info is a little harder to find out there.

[WorkinLonghorn]

I'm wondering if the numbers I posted in my first post would tell us anything.

I'll look it up for you the next time I go by the "Auto Books" store in Burbank. There's a book there called "Chevy ID Numbers" Or "Chevy by the Numbers".
Personally I am not familliar with the differences in appearance or in how to ID transmissions etc. I'm somewhat familiar with auto-trans stuff because I used to manage a couple AAMCOs way back when.-cougar

[Brad]

It would be much easier to identify if you posted a picture of your transmission. We could tell you exactly what you have. And unless someone put in a Muncie car transmission, there's no way you have a 2.42 1st gear.