The Stovebolt Site Forums The Shop Area Engine Shop What is rear end gear ratio?

I would like to find out what the rear end gear ratio is in my truck. One friend says it might be a 373.

How can I find out? Is there a way to determine this without rmoving the differential cover? Please explain the procedure. If the cover must be removed to get accurate gear ratio, how is it done?

Is there a site that can explain this to me?

Thank you!!

what year/size is the truck, and do you think it's the original rear? pretty much all the 1/2T's had 3.90 rears, 3.73 would hafta be a replacement unless it's a newer truck

Bill

this previous post may help.

click here

I thought the stock rear differential gear ratio on the 47-55 1st 1/2 tons was 4.11:1?

The year and model of your truck is a good place to start in terms of figuring out what the gear ratio is. Then you can look in the shop manual at the chart showing gear ratios for different models.

The method suggested in the thread posted by Joker might be difficult to implement if you've got a torque tube rear end. The way to do it with an open driveline is to count the number of drive shaft revolutions it takes to get the tires to go around once. Usually you work with 10 revolutions of the tire and move the decimal point. However, since in a torque tube (aka closed driveline) you can't see the drive shaft is turning that way won't work.

If you do it by pulling the cover to the differential you would count the number of teeth on the ring gear and the number of teeth on the pinion gear and then divide. Many times the ratio is stamped into the ring gear in that format- 11 pinion teeth and 41 ring teeth = 3.72:1. The ring gear would be stamped with several numbers including 11 (space) 41.

According to the Standard Catalog of Chevrolet Trucks, the '54/'55 1st 1/2 tons had 3.90 gears, '53 and earlier AD's had 4.11's. There should be an ID code stamped on either the 3rd member or axle tube (depending on year/axle type) that can be de-coded to tell you the ratio. Cars & Parts has an exellent id guide that has the numbers for '46-'72 trucks, it's the Catalog of Chevy ID Numbers 1946-1972. Very informative.
Re-read your post. On a '65 1/2 ton the standard ratio was 3.73, with 3.07 and 4.11 optional. The codes are; 3.73-HA,HC,HE,HF,HG,HQ;3.07-HB;4.11-HD,HH. Hope this helps you.

So sorry, I neglected to add this info. This is a 65 Chevy C10. I am pretty sure it came with a 292. It now has a 350. It appears to have original type clutch and transmission. So I am assuming the rear ends is the original.

According to http://www.drivetrain.com/GMtrkratio_posidata.html The truck has a "GM 8.875 12 bolts 11inches wide".

Longbox55, I did not know about the codes. I will try to find that code on the truck. Thanks.

There was a lot of helpful info in here. Thanks to you all. Hopefully, I have enough to be able to get the gear ratio.

The axle ID code should be stamped on the right axle tube out towards the brake, sometimes on the front mostly on the back. You will need to wire brush the and possibly sand the area till you find the two letter code stamped in the steel tube. The will be about 3/8" x 3/8" and not stamped very deep. Good luck hunting! Joe

The only number I could find today is "F254". On upper rear edge of differential, right in front of cover. Anyone know what it means? Probably is not what I need though. Have not found a two letter code yet. Will look again, hopefully tomorrow.
Thanks.

I have a '64 that is 100% original. The ORIGINAL owners manual lists 3.73 gears as standard with 4.11 optional. I imagine the '65 year was similar.

The F254 looks like a date code, June 25, 1964. This would be right for a '65 truck. Keep looking out on the axle tube close to the brake backing plate or around the lower control arm mounting. Joe

Thanks for all your suggestions and help. It ended up sounding like the easiest way to do this was to just spin the tire and count revolutions of the drive shaft. I did like NM51 suggested and spun the tire and counted the revolutions of the drive shaft. It worked out to be the 3.73.

Thanks again for all of your suggestions and time. I really do appreciate it!!!!!!!!!!!!!