Is there an easy way to determine my gears? would like to install overdrive transmis

[327-C10]

I would like to determine my 68 C-10, 1/2 ton's rear gears. Aside from opening it up and counting teeth is there an easyer way to tell the gearing.

Any chance any of the codes on the inside of the glove box doors show the ratio. The truck is a 307, 3 speed.

Has anyone tried one of these overdrive (manual) 5 speeds. What is the preferred rear gears with this type of set up. When I'm done I would like it to be a daily driver good enough for the Friday night cruises.

Thanks in advance for any comments.

[gonzo6869]

Do you have a tach on the truck? If you do then what is the RPM while at freeway speeds. I have 3.73's and with 275/60 15 all around I am pushing 2800 RPM around 60 MPH.

[Pop's C-10]

Jackup one side of tha truck...mark tha rear end yoke with something you can see..rotate one wheel & count how many times tha mark passes..that's get you a good idea

[raycow]

Originally Posted by Pop's C-10

Jackup one side of tha truck...mark tha rear end yoke with something you can see..rotate one wheel & count how many times tha mark passes..that's get you a good idea

Almost, but not quite. Turn the wheel TWO revolutions and count how many times the driveshaft turns, including any fraction of a revolution. That number is your axle ratio.

If the wheel is difficult or impossible to turn you may have posi. In that case, jack up both wheels and turn either of them ONE revolution.

Ray

[raycow]

Originally Posted by 327-C10

Has anyone tried one of these overdrive (manual) 5 speeds. What is the preferred rear gears with this type of set up. When I'm done I would like it to be a daily driver good enough for the Friday night cruises.

I installed a T-5 5 speed in my 60, which came with 3.90 gears, and I am very satisfied with its performance in overdrive. 3.90 wasn't my preferred gear, but in my case it wasn't practical to do a rear end swap without any welding. I have a 305 and it feels like it could pull a taller gear than that without any problems.

Ray

[scrapmetalart]

here is a code stamped on the front of the passenger side axle tube. Then google

[327-C10]

Thanks for the help. I think I'll start by spinning the wheel(s). Who knows if the original gears are still in the rear. I will look for the code on the axle housing. No Tach yet.

Thanks all.

[gkesseru]

Here's method to get it exact:. make a mark (via masking tape and magic marker) on each of rear yoke and diff housing. I put a piece of masking tape around the entire tire and make a start mark using a chair or something to point to the start mark on the tire. Turn the wheel until the drive shaft makes exactly one turn. Make a second mark on the tire masking tape for the distance the tire traveled. Cut off the tire masking tape exactly at the start mark. Lay the tape straight and measure the tire circumference (whole tape) and divide by distance tire moved (second mark). Multiply by two. That's the ratio. Don't multiply if you have posi.

[raycow]

Now that's creative thinking. You are dividing two straight line measurements, which may be more accurate than trying to guesstimate a fractional driveshaft revolution. You are the first person I have ever heard describe this method, and I like it!

Ray

[Kidd-7]

Question: how does the tire size play into that equation?

[68350]

kidd-7, yes the taller the tire the lower(numerically) the gear ratio will FEEL.

[Ronnny]

kid7

tire size plays a lot into what ratio you want in your rear. also has to do with your motor and tranny. what speed you want to be at when doing your usual driving. the speed you drive at most of the time you want the motor to be in its powerband. lower rpm does not always mean better miles per gallon. for a good easy calculator go to ringpinion.com and it will give you a lot of information. to make up for to low a gear you can run taller tires but most of us do not want to do that. Or for to high of a gear for what you do you can run a smaller tire.

[cdowns]

removing the cover cleaning everything up and adding freash gear oil in place of 40year old sludge is a worthwhile progect a good way to spend an hour///whgile you're at it you can make sure of your ratio

[327-C10]

Once again thanks all.

[gkesseru]

Raycow: My 36 years as an engineer is showing. One day I was staring at the frame up on jack stands and thought that there must be a more easy and accurate way to measure than to guess at the number of drive shaft revolutions. I came up with this and was surprised to end up with a result within 1-2% of actual, without taking off the pumpkin cover.
P.S.: Make sure the other wheel is not turning.

[raycow]

Gkesseru, I am an engineer too (now retired), and I have been working on cars, trucks and motorcycles since before I was old enough to drive. As I said, I have never read about your method anywhere else before now. Thank you for sharing it.

Ray