Rear End Gears

[widewhite70]

Before I take the pan off of my rear end to figure out what i've got I was wondering if anyone would know what i've got. It was originally a straight 6 with a 3 on the tree, its a 12 bolt 6 lug. Any ideas?

[LMN8UR]

Can you put the rear up on jack stands. Use chalk and mark the bottom of the tire and the drive shaft. rotate the rear wheel and count the shaft revolutions.

In '72 the standard for a 1/2 ton was a 3.73 rear.

[cdowns]

pull the cover off use a can of brake cleaner a new gasket and two quarts of grease is a good afternoon maintence job to do

[widewhite70]

So i'll do one revolution of the tire and count how many times the drive shaft turns? Is there a math formula I'll need to do?

[widewhite70]

I'm wanting to change the gears, I've got a 327 with a T-10 4 speed and at 60 I'm taching like 2,600 RPM so I want to get it a little more highway friendly

[LMN8UR]

Originally Posted by widewhite70

I'm wanting to change the gears, I've got a 327 with a T-10 4 speed and at 60 I'm taching like 2,600 RPM so I want to get it a little more highway friendly

What size tires do you have?

[LMN8UR]

If you have 235-75-R15 tires, and a 3.73 rear gear you should be turning 2,606 rpm at 60 mph.

[widewhite70]

I've got 275/60/15 on the rear

[LMN8UR]

Common gears are =

3.08:1 = 2,152
3.43:1 = 2,397
3.73:1 = 2,606
4.10:1 = 2,865

[LMN8UR]

Common gears are = w/275-60R15

3.08:1 = 2,220
3.43:1 = 2,473
3.73:1 = 2,689
4.10:1 = 2,956

But if the speedo gear wasn't changed when you put those tires on your speedometer is reading fast. So When you think you're going 60 you are really only going about 57.

[widewhite70]

Yeah that is true I never changed the speedo gear, but thanks for all the help! I think I'll just end up pulling the cover to know what I have for sure.

[GuyO]

Hi, if you don't want to crawl under the truck you could look on your glove box inside door is a RPO code which will tell you your VIN and specific vehicle options when new. However, if modifications have been made including custom tires or overdrives (200-r4, 700-r4 transmissions) the change in speedometer and tach speeds can be effected greatly. Attached below is a table I created based on 235/75R15 tire diameter, rear gears, etc. If anyone uses the table or recreates it, please keep in mind that if you have an overdirve 4th gear, then you must multiply your rear differential gearing by the percentage of the overdrive. Example 3.73 X .67 (4th gear 200-r4) = your effective gear reduction is 2.50. I hope this helps, Guy.

[domeier]

Guyo: I don't understand why you show two different tire heights for 235/75R15 tires, re. 28.9" and 27.7"?

[GuyO]

Hi, yes I should have explained that. The original tires equipped on the C-10, C1500's were "G"s. The tires used in the calculation were p235/75R15. There is a little difference in tire height. Enough to run the speedometer slow by 4%! Sorry, I should have clarified better! Guy.

[Longhorn Man]

There is more often than not, no gear listed on the S.P.I.D. This is due to the fact that it only lists options. The confusing part is figuring what gear ratio came with the engine and tranny that came in the truck.
A half ton truck running an inline 6 with a 3 speed with no overdrive would have had a 3.73 ratio.
Posted via Mobile Device

[bucktruck68]

Originally Posted by GuyO

Hi, if you don't want to crawl under the truck you could look on your glove box inside door is a RPO code which will tell you your VIN and specific vehicle options when new. However, if modifications have been made including custom tires or overdrives (200-r4, 700-r4 transmissions) the change in speedometer and tach speeds can be effected greatly. Attached below is a table I created based on 235/75R15 tire diameter, rear gears, etc. If anyone uses the table or recreates it, please keep in mind that if you have an overdirve 4th gear, then you must multiply your rear differential gearing by the percentage of the overdrive. Example 3.73 X .67 (4th gear 200-r4) = your effective gear reduction is 2.50. I hope this helps, Guy.

Great info.