Jack up one wheel, make a mark on pinion flange and brake drum to backing plate on wheel in air. Turn wheel in air one complete turn while counting the number of times the pinion turns. Multiply this number times two equals ratio. The choices are 3.73, 4.10, 4.56

The ratio should be stamped on the ring gear also. Divide number of teeth on ring gear by teeth on pinion for ratio
41/10 =4.10

 

It's easier to look in the glove box or console at the option RPO codes:

G72 : AXLE REAR, 2.14 RATIO
G74 : HEAVY DUTY SUSPENSION
G75 : AXLE REAR, 3.62 RATIO
G76 : POSITRACTION REAR
G80 : AXLE POSITRACTION, LIMITED SLIP
G81 : POSITRACTION REAR AXLE
G82 : AXLE REAR, 4.56 RATIO
G84 : AXLE REAR, 4.10 RATIO
G86 : AXLE, REAR, LIMITED-SLIP
G87 : RING GEAR, 8.50 INCH
G89 : RING GEAR, 7.50 INCH
G91 : SPECIAL HIGHWAY REAR AXLE RATIO 3.08
G92 : AXLE REAR RATIO, PERFORMANCE
G94 : AXLE REAR, 3.31 RATIO
G95 : AXLE REAR RATIO, ECONOMY
G96 : AXLE REAR, 3.55 RATIO
G97 : AXLE REAR, 2.73 RATIO
GH0 : AXLE REAR, 3.54 RATIO
GH2 : AXLE REAR, 2.29 RATIO
GH3 : AXLE REAR, 2.77 RATIO
GH4 : AXLE REAR, 2.92 RATIO
GK9 : AXLE REAR, 4.63 RATIO
GL3 : AXLE REAR, 6.17 RATIO
GM1 : AXLE REAR, 2.59 RATIO
GM3 : AXLE REAR, 3.45 RATIO
GM8 : AXLE REAR, 2.56 RATIO
GMC : PLANT CODE PONTIAC, MI, USA
GN9 : AXLE REAR, 4.11 RATIO
GQ1 : AXLE, STD RATIO
GS1 : AXLE REAR, 2.73 RATIO
GS3 : AXLE REAR, 3.73 RATIO
GS4 : AXLE REAR, 3.70 RATIO
GS5 : AXLE REAR, 4.11 RATIO
GS6 : AXLE REAR, 4.56 RATIO
GT1 : AXLE REAR, 2.56 RATIO
GT2 : AXLE REAR, 2.29 RATIO
GT4 : AXLE REAR, 3.73 RATIO (DUP WITH 5 X 1)
GT5 : AXLE REAR, 4.10 RATIO (DUP WITH GT8)
GT7 : AXLE REAR, 3.33 RATIO
GT8 : AXLE REAR, 4.10 RATIO (DUP WITH GT5)
GTY : AXLE, WIDE TRACK
GU1 : AXLE REAR, 2.41 RATIO
GU2 : AXLE REAR, 2.73 RATIO
GU4 : AXLE REAR, 3.08 RATIO
GU5 : AXLE REAR, 3.23 RATIO
GU6 : AXLE REAR, 3.42 RATIO
GV1 : AXLE REAR, 2.73 RATIO
GV2 : AXLE REAR, 5.83 RATIO
GV3 : AXLE REAR, 3.08 RATIO
GV4 : AXLE REAR, 3.36 RATIO
GV5 : AXLE REAR, 3.55 RATIO
GV7 : AXLE REAR, 4.11 RATIO
GV8 : AXLE REAR, 2.72 RATIO
GW2 : AXLE REAR, 2.56 RATIO
GW3 : AXLE REAR, 2.56 RATIO
GW4 : AXLE REAR, 3.31 RATIO
GW5 : AXLE REAR, 2.73 RATIO
GW6 : AXLE REAR, 3.27 RATIO
GW8 : AXLE REAR, 4.10 RATIO
GW9 : AXLE REAR, 2.93 RATIO (DUP WITH GU3)
GWL : SPRING, RR, 9000 LBS CAPACITY
GX1 : AXLE REAR, 3.70 RATIO
GX2 : AXLE REAR, 3.07 RATIO
GX3 : RATIO,TRANSAXLE FINAL DRIVE 3.33
GY2 : AXLE REAR, 3.31 RATIO
GYS : RATIO TRANSAXLE FINAL DRIVE 3.65
H01 : AXLE REAR, 3.07 RATIO
H04 : AXLE REAR, 4.11 RATIO, SINGLE SPEED
H05 : AXLE REAR, 3.73 RATIO
H12 : AXLE REAR, 21000 LBS, EATON 21065S, SINGLE SPEED
HA3 : AXLE REAR, 5.29 RATIO, SINGLE SPEED
HC4 : AXLE REAR, 4.56 RATIO
HC7 : AXLE REAR, 5.13 RATIO, 7500 LBS, SINGLE SPEED
HC8 : AXLE REAR, 3.21 RATIO, SINGLE SPEED, TRUCK
HE3 : AXLE REAR, 3.07 RATIO, 3500 LBS, SINGLE SPEED,
HF7 : AXLE REAR, 4.56 RATIO, 10000 LBS, DANA 70, SINGLE
HF8 : AXLE REAR, 4.88 RATIO
HK1 : AXLE REAR, 2.87 RATIO
HK9 : AXLE REAR, 5.86 RATIO 10000 LBS, SINGLE SPEED

 

What's the type of axle where both wheels turn in the same direction when the truck is raised and in neutral. Don't those axles have to have both wheels in the air and then you count the total # of drive shaft revolutions?

 

limited slip. only luck i had figuring them out was too read the ring gear numbers

 

Another thing you can do is put your VIN into a VIN decoder. Someone had one posted a while ago. That assumes tho that your axle is original.

 

If you have a clutch type limited slip or a detroit locker, simply jack up both rear wheels, make a mark on the brake drum and another mark on the pinion, turn pinion until wheel makes one complete turn, 3 3/4 turns would be 3.73, 4 turns would be 4.10 and so on.

 

Seems like a lot of work to pull the diff apart. Turn the wheels and it will tell the story. Just watch and do it a few times to be sure.

 

Also why would you turn the pinion. Much easier to turn the wheel

 

Also why would you turn the pinion. Much easier to turn the wheel

If it is a clutch type limited slip, and you turn a tire and the other tire doesn't turn at the exact same speed, (ie clutches aren't real tight) then you will get a false reading. Turning the pinion or driveshaft has a much better chance of turning both tires together with a limited slip. Again, you can only do it this way with a tight limited slip or detroit locker. This won't work with both tires jacked up with a gov lock.

 

i always say it and ill say it again... never trust the vin or some glovebox code unless you are the original owner of the truck. gears can be changed and so can rears real easy. i dont even like the wheel and driveshaft way, just so simple to pull the cover off and count some teeth, plus you get to see how its all doing in thurr.

 

i always say it and ill say it again... never trust the vin or some glovebox code unless you are the original owner of the truck. gears can be changed and so can rears real easy. i dont even like the wheel and driveshaft way, just so simple to pull the cover off and count some teeth, plus you get to see how its all doing in thurr.

x2. rear fluid is neglected. the teeth count is stamped on the side of the ring gear read it.

 

Ok

 

x3 on pullin the cover...differential fluid is one of the most overlooked fluids in the vehicle. If it does happen to be a posi (clutch locked) differential, its that much more important to keep it serviced. You don't actually have to "tear the rearend apart", just get some good gear oil and quality silicone, pull the rear cover off, get your ratio off the ring gear, clean up and reinstall the cover and put your fresh gear oil in. There, you know exactly what the ratio is and you just serviced your differential which should help you sleep better at night knowing that that is one fluid that does not need changed for a while and that it is topped off. Good luck

 

Anyone have any experience with a differntial plug that is like a large Robertson screw. My last 2 trucks had square bolt heads but this one is the opposite. what do I need to get to open the plug?

 

Ok, I don't know what a robertson screw is, but if I'm understanding you right, a 3/8" ratchet and short extension should do the job. Good luck

 

Oh yeah i forgot about Americans and their phillips bits. Robertson is a square bit. So you don't use any sort of socket just the extension? Never would have thought of that. thanks. I guess if 3/8 is too small then go to 1/2"

 

It should be a 3/8'' socket for the plug.....

Finally we pulled all three covers, two were 4:10's and one was a 3:42. We will try the 3:42. This and one of the 4:10's were limited-slip, still looking for a dually to swap in.....

The pinion in the 3:42 was 12 teeth, instead of 10 like the two 4:10's. we now have the other two boxed up as spares, as well as all of the good brake parts.

The differentials are really an over looked piece for changing out the oil, so when we get our dually we will install a drain plug in the bottom and possibly a temperature probe in the oil.... thanks for all of your aid.

 

where do i get the additive for the factory locking diffs? mine is probably well over due for a change. and i wont be searching for a 14 bolt rear diff for a while so this 10 bolt will have to live for a bit longer.

 

I'm not sure who ALL carries the stuff, I've gotten it from the GM dealer, but most of the time I get it at NAPA. I would recommend using synthetic gear oil in it as well, as clutch locked differentials run much hotter than an open differential. The higher heat tolerance of the synthetic oil will make it last MUCH longer.

 

its available at any of the part stores around here

 

Most of the synthetics do a great job in clutch type limited slips.