Rear end question

[Green79k10]

I was curious if anyone could give me some info on the numbers stamped on my rear end.. mainly curious if you can tell the gear ratio from it.. it’s a corporate 12-bolt on a 79 k10.. I’ve researched the heritage site.. and I can’t find any matches

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[Octane]

I believe my 77 has the same numbers.I know mine is a 3.73:1There may be a tag still on your diff cover with the axle ratio

 

[Swearbody]

Ive yet to see one have the gear ratio listed anywhere other than rpo codes before 1999 year model. After 99 they sometimes stamped the rpo code for the axle on a tag bolted to the diff. Dont know if they did that earlier.
Im not a rear end expert or anything but ive had a corp 12 bolt and never found the ratio listed.

Have you looked in your glovebox for the rpo sheet.

 

[BKING33]

Those numbers r casting numbers. you will have to look at the right axle tube to find the code. it is stamped on frt of the tube about 3-5" outboard of the center carrier housing. From my experience it can be quicker to pull the cover than spend 30 minutes trying to clean crude and rust off of tube.

 

[hoagster]

@BKING33 He is correct you need to find the codes on the right axle tube, and if it has never been taken apart and upgraded it will be correct. Best bet is to take the diff cover off and count the teeth. I added pictures of my build so you can compare. Clean the snarf off and rebuild it. It probably needs new seals axle and pinion seals.

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[hoagster]

when your done you will be happy! There is a break down of the axle codes on this site which will tell you what you have.

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[75gmck25]

The rear axle in my '75 had a small metal tag bolted onto one of the cover bolts and it listed the 4.10 ratio. However, my guess is that these tags are long gone on most axles

Bruce

 

[Green79k10]

Thanks for the info guys!

 

[SirRobyn0]

I was curious if anyone could give me some info on the numbers stamped on my rear end.. mainly curious if you can tell the gear ratio from it.. it’s a corporate 12-bolt on a 79 k10.. I’ve researched the heritage site.. and I can’t find any matches

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Thanks for the info guys!

I've got two ideas for you. #1. it's usually not to hard to pop the cover and count the teeth to know 100% for sure what the axle ratio is. Even if there is a tag some place who's to say someone along the line didn't re-gear it.

#2. This is the true lazy man way to figure out your gear ratio, or close to it. You have to have a tach, and a reliable way of telling mph, I like to use a GPS, because it gives a digital reading. Having an assistant is nice, but it's doable by yourself on a empty road if your willing to make a couple passes.

Drive it in drive or top gear, (not overdrive) you want 1 to 1 ratio, to an higher but arbitrary RPM I like to use 3,000RPM, you could use 2,500 or 2,000 if you need to bring the speed down. If you have an automatic and it does not have torque converter clutch you'll need to over shoot the RPM you choose to hit by about 150 RPM. Assuming you have picked 3,000 RPM, remember the speed you were going when you hit that RPM, for the sake of discussion let say it was about 67MPH. Then use this calculator https://spicerparts.com/calculators/transmission-ratio-rpm-calculator You will need to take guesses at the ring and pinion gear ratios, but we know 3.42, 3.73, 4.10 and 4.56 are the most common. So you know the height of your tires, you know the RPM and the transmission ratio, punch that info in then start taking guesses at the ring and pinion. Using my made up data if your tires are 31 inches tall, you'd have a 4.10 rear end.

 

[Octane]

How about mark the tire and turn one revolution as you count the number of times the driveshaft turns?

 

[Ricko1966]

The fastest easiest way to do this is tape a piece of string to the driveshaft, tie a nut on the end for weight. Mark your tire ,push it 1 revolution.How many times is the string wrapped around the driveshaft . If you want to do the math on string length vs. Driveshaft diameter you can get right on but visual is enough for me close to 3 close to 3 and a half close to 3 an three quarters close to 4.

 

[Octane]

If anyone wants to do a funny prank how about putting a long zip lock tie on the driveshaft and when they drive away....wap wap wap and they will think the car is really messed up.As you laugh at them and make fun of their screwed up car. Lol. I have a rather dark sense of humor I suppose.

 

[SirRobyn0]

If anyone wants to do a funny prank how about putting a long zip lock tie on the driveshaft and when they drive away....wap wap wap and they will think the car is really messed up.As you laugh at them and make fun of their screwed up car. Lol. I have a rather dark sense of humor I suppose.

No it's funny for sure, however years ago I worked at a shop, a guy came in with his truck making noise because someone did this to him. We told him what it was (didn't charge him) and he then informed us he had just been over inspecting the asphalt plant, turned out he was a contract inspector for the state, told us he was going back the next day to finish his inspection, I bet they got looked at really closely the next day as he was not the least bit amused.

 

[TubeTruck]

Why count the teeth? Just look at the stampings on the ring gear. 10 41, so 41/10, it's got 4.10's

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