Where can I find my rear end gear ratio?

[1979C20]

Thats the stock tires. My options sheet doesnt say my gear ratio either...

[Stewart G Griffin]

Stew, I think Fitz is correct with his statement.  The older these vehicles get, the more likelihood of having parts interchanged.  I have knonw many of our trucks get the entire axle swapped.  When going to salvage yards, the first things stripped from these trucks are the axles/gears.  Every 73-87 p/u out in the graveyard no longer has its rear gears.

It's highly unlikely that the rear gear has been swapped.  

                                                                                                                                                                                           why is that? Someone already pulled the 305 and installed a built 350. When your dealing with an old modified vehicle anything goes. Keep in mind he is looking to add a limited slip carrier to his axle. In order to buy the correct limited slip unit you must know your gear ratio.

Because it's too complicated to change the rear gears and it's too much hassle to change out the entire rear axle assembly--- and for what?  It's not like you're going to racing with one of these.  At least not with a non-posi 3.73.

i would be willing to bet money it has not been changed from stock.

[jbell1]

So pulling off the cover is the only way to know?

[Grim 82]

Pulling the cover is the guaranteed way to find out what you have.

Swapping out the entire differential is not that much of a hassle, and most of the trucks I have seen, including mine, have been swapped at some point. Example; the original rear pinion gear on my k10 exploded, and it was cheaper to buy a parts truck with a good axle than to have a new r&p set up for it, so I swapped it out. Because it's a 4wd I had to swap the front too so they would match. In the next month or so I will be swapping both axles to get the lower ratio I want from my suburban parts truck now that I am running taller tires. That will be the third rear end running under just this one particular truck. In other cases, people will swap in a 12 bolt, or upgrade to 3/4 ton running gear for strength. When the 3.08's come out of my truck they will be going in a friend's 1980 k10 to replace the 2.73's. From what I have seen not only is it common but practically guaranteed that the axles have been swapped.

If yours has the original equipment I would say that's the exception, not the rule.

[fitz]

i would be willing to be money it has not been changed from stock.[/b]

Growing up my parents told me I could be anything. I never thought of being money. Sorry Stew, couldn't resist.
All I'm saying is that if it was my truck and I was going to spend $500 or more on a limited slip for it I would pull the cover to make sure I order the correct parts.

[jbell1]

Okay thanks, but isnt there a catalog somewher that should tell me what the rear end is online? I've tried looking but cant seem to find one anywhere, it is a 3/4 ton axle so i'm thinking its a high ratio, I just wanna know for sure what it is before I go swapping stuff into it.

[1979C20]

3/4 tons usually had lower ratio for towing. Especially with a 305, it would need lower gears to get 'er goin. Im a little shocked it had a 305 from factory. Usually most 3/4's ive seen have 350's. If you look.at my rpo sheet i posted earlier, it says "5.7 litre (350cid)". Does your say 5.0?

[jbell1]

yeah it says 5.0 305, it also says 5.0 on the radiator bracket sticker.

[zieg85]

I almost bought a 1986 C-20 with a 350 4 speed that had a 3.42 but only wound up with the rf fender and some misc. pieces.

[Blazin]

I have an 84 C20 with a 305, TH 400, and a semi floating rear end with 3.42 gears. All original. I got it from the original owner.

[jbell1]

okay I think rather than pulling off the cover I'll just bring the vin number to a dealer and see what i got for a rear end I just really dont wanna screw shoot up when i open it.

[autorepr]

Another way to tell what gear is in the axle without pulling the cover is to mark the driveshaft at say 3 oclock and then the axle housing on the pinion snout so the 2 marks align, then mark one of the wheels at say the 6 oclock position. Have someone roll the truck forward while you watch the mark on the drive shaft(needless to say...DON'T BE UNDER THE TRUCK look from the side out of the path of the wheels) and count how many times the marks pass each other and stop when the wheel makes 1 complete revolution. (wheel mark is back at 6 oclock) Guesstimate the last fractional turn. 1/4 turn would be .25, half turn would be .5, 3/4 turn is .75 then add the full driveshaft revs to it. Like- 2 full turns plus not quite a 1/4 turn would be 2.23-1,  3 full turns plus not quite 3/4 turn would be 3.73-1 etc.
If you have help, that would be quickest and easiest.

[jbell1]

Wow never thought of that haha I'll probay do that it sounds pretty easy. When I do it I'll post what I get so you guys can help me figure out the exact ratio

[1979C20]

Instead of rolling the truck, save yourself some energy, and jack the wheel off the ground.

[Laderhosen]

Taking the diff cover off and seeing what is written on the ring gear would surely be the easiest option?!? It takes half an hour max! All you need is one socket and an oil tray.

Once you've taken the diff cover off rotate the crown wheel until you see 8 numbers stamped into the edge, in groups of 2. I think mine said summin like:

12 41 11 87

This means 12 teeth on the pinion, 41 on the ring gear, 11 - month of manufacture, 87 - year of manufacture.

41 divided by 12 equals a 3:42 for me.