Rear End Gears 75 GMC 4x4

[qboy]

I'm trying to find out what the stock rear end gear ratio was for a 1975 GMC C15 4x4 SWB Fleetside.

[werewolfx13]

It varies from truck to truck, I'm not familiar with the 70s models enough to venture a guess as to what was most common, but the best way to find out is to pop the diff cover off. Rear ratio will most likely be the same as the front, unless somone has done somthing REALLY stupid..

[Fordeatinz71]

well, there is a couple ways to go about this.  if you are fortunate, you have the option sticker SOMEWHERE on the truck (usually either inside the glove box lid or on the fenderwell under the hood) and all you have to do is look on there and it tells what gears you have (unless you're like me and it says "Rear Axle-Standard"-then just get the two letters and number beside it, the code, and post it on here).

if you DON'T have the RPO sticker, you have two options, one of which i boycott because it's just too much darn work.  you COULD pop off the differential cover like most people on here mention...getting fluid everywhere and then having to count a bunch of teeth (unless you are lucky enough to still be able to see the stamp that SHOULD have been on there...read SHOULD...but hey, the General never makes mistakes, does he?) and do some math to figure it out.  then you'll have to reseal it and refill it with fluid.  i guess this way is good if you need to service the rear end anyway (if you can't remember when it's been serviced last, you should do this way)

if you keep up with your service, and/or just don't WANT to do all that, you can do it my way.  here's how.  you jack up the rear end (CHOCK THE FRONT WHEELS, YOU WILL HAVE THE TRUCK OUT OF GEAR AND OFF THE GROUND, EBRAKE ONLY HOLDS THE REAR WHEELS-WHICH WILL BE OFF THE GROUND...that's my safety spiel), get some chalk or something that will mark on metal.  make a mark on the pinion yoke (the part your driveshaft connects to on the rear axle, that spins with it) and then on the housing to annotate where the pinion is sitting at.  then make a mark on the tire and on the body (inside of the tire, we don't wanna leave marks on the visible body of the truck, now do we?).  slowly roll the wheel until it has rotated one full turn (this is why you need the marks, unless you are good enough to remember exactly where one turn is).  as you are rotating the wheel, you need to be watching the driveshaft, and counting how many times it goes around.  that is your axle ratio.  here's an example...if your driveshaft spins around 3 times and lands just BARELY past your reference mark then you probably have 3.08 gears.  if your driveshaft doesn't spin QUITE 3 times, but almost does, then you most likely have 2.73 or 2.76 gears. 

this method is NOT exact.  the only EXACT method is to pop the cover and look for the stamp in the gear or to count teeth and divide the pinion number by the ring number.  but this will get you really close if you do it right.  and given that the options for most given years are pretty far apart (i.e. for '75 your options were as follows, 3.07, 3.40, 3.73 and 4.11-don't know how accurate that info is, i got them out of a '75 Chevy trucks brochure...might have been different ones, i just reprinted what they had...GMC didn't advertise gear ratios in their '75 brochure...) so you will be able to basically get exact unless your gears were changed. 

you're lucky in one sense.  you got in before they did the SUPER tall gears like they did in the late 70's-mid 80's.  my poor '83 Sierra 4x4 was crippled with factory 2.73 gears.  which if you don't know are HORRIBLE gears for low-end and TOO tall of a gear for ANYTHING with an overdrive transmission, which i have. 

good luck!

[qboy]

well, there is a couple ways to go about this.  if you are fortunate, you have the option sticker SOMEWHERE on the truck (usually either inside the glove box lid or on the fenderwell under the hood) and all you have to do is look on there and it tells what gears you have (unless you're like me and it says "Rear Axle-Standard"-then just get the two letters and number beside it, the code, and post it on here).

if you DON'T have the RPO sticker, you have two options, one of which i boycott because it's just too much darn work.  you COULD pop off the differential cover like most people on here mention...getting fluid everywhere and then having to count a bunch of teeth (unless you are lucky enough to still be able to see the stamp that SHOULD have been on there...read SHOULD...but hey, the General never makes mistakes, does he?) and do some math to figure it out.  then you'll have to reseal it and refill it with fluid.  i guess this way is good if you need to service the rear end anyway (if you can't remember when it's been serviced last, you should do this way)

if you keep up with your service, and/or just don't WANT to do all that, you can do it my way.  here's how.  you jack up the rear end (CHOCK THE FRONT WHEELS, YOU WILL HAVE THE TRUCK OUT OF GEAR AND OFF THE GROUND, EBRAKE ONLY HOLDS THE REAR WHEELS-WHICH WILL BE OFF THE GROUND...that's my safety spiel), get some chalk or something that will mark on metal.  make a mark on the pinion yoke (the part your driveshaft connects to on the rear axle, that spins with it) and then on the housing to annotate where the pinion is sitting at.  then make a mark on the tire and on the body (inside of the tire, we don't wanna leave marks on the visible body of the truck, now do we?).  slowly roll the wheel until it has rotated one full turn (this is why you need the marks, unless you are good enough to remember exactly where one turn is).  as you are rotating the wheel, you need to be watching the driveshaft, and counting how many times it goes around.  that is your axle ratio.  here's an example...if your driveshaft spins around 3 times and lands just BARELY past your reference mark then you probably have 3.08 gears.  if your driveshaft doesn't spin QUITE 3 times, but almost does, then you most likely have 2.73 or 2.76 gears. 

this method is NOT exact.  the only EXACT method is to pop the cover and look for the stamp in the gear or to count teeth and divide the pinion number by the ring number.  but this will get you really close if you do it right.  and given that the options for most given years are pretty far apart (i.e. for '75 your options were as follows, 3.07, 3.40, 3.73 and 4.11-don't know how accurate that info is, i got them out of a '75 Chevy trucks brochure...might have been different ones, i just reprinted what they had...GMC didn't advertise gear ratios in their '75 brochure...) so you will be able to basically get exact unless your gears were changed. 

you're lucky in one sense.  you got in before they did the SUPER tall gears like they did in the late 70's-mid 80's.  my poor '83 Sierra 4x4 was crippled with factory 2.73 gears.  which if you don't know are HORRIBLE gears for low-end and TOO tall of a gear for ANYTHING with an overdrive transmission, which i have. 

good luck!

Very nice explanation with the chalk technique.   I'll give that a try.   No way I'd pull the cover off.   Thanks.

[Fordeatinz71]

no problem man.  good luck and let us know what you find.  post some pics of that truck when ya get a chance...we never get too many pictures on here!