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rear axle ratio RPO

453K views 424 replies 263 participants last post by  Rhotpursuit 
#1 ·
GU4=3.08
GU5=3.23
GU6=3.42
GT4=3.73
GT5=4.10
HC4=4.56

G80 - locking differential


compare to the codes in your glove box to know which gear ratio you have :)
 
#211 ·
Can anybody tell me the correct way to check rear ratio w/o removing diff cover and my RPO is missing and I have another rolling chassis that Im putting a 1935 chevy sedan on top of. One Chassis is a 1985 w/ 2.8/5speed ext cab and the other is a 1987 w/ 2.8/5speed std cab long bed. Any help is appreciated
 
#217 ·
Good question. I'm wondering the same thing myself. If the glove box is part of a spongebob square pants lunch pail somewhere, and the S10 or Blazer has been melted down and turned into fence posts, how can you determine what gear ratio you have without taking the cover off? A junkyard prob isn't going to let you rip a cover off to see. So, did GM stop putting codes on the axle tubes? If not, where are they located and how do you decipher them? Simple question.

Anyone?
 
#214 ·
I don't know about finding out the gear ratio without taking off the cover, but I can tell you how to tell the ratio if you do take it off. It's very simple. You count the number of teeth on each gear and then divide the output by the input.

I had a question, though, I have a GU6, also, open diff. I'm going to go junk yard diving to find a G80 soon. Will it have the GU6 and G80 in the glove box? Cause my truck's 4wd and I don't want to fry my T-case by having different ratios up front and in the back.
Also, do all second gens fit on all second gens? My truck's a 96, will I be able to but a G80 from a 98-99 in mine?
 
#220 ·
so, my glove says i have a 3.08, but a previous post on here said that with the 4.3 that was only an option on the 5spd models....what is up with that?
 
#222 ·
I think I know the RPO code for my truck now. I just want to make sure before I go and buy a new rear end for my truck. on one side of the chart in my glove box it says 6HO4 and directly over in the next column it says rear end 4.11 ratio. Any help would be appreciated.
 
#231 ·
spin the axle shaft and count how many times it turns and count when the pinion rotates one time...

so if it pinion yoke thing spins one full turn and you spin around your axle 4.10 times...then you got 4.10's

You got that backwards. Easy to get that confused. The pinion will turn 4.1 turns to the Axle/Wheel turning once. That also doesn't work well on open differentials because the spyder gears come into play. The other wheel needs to be turning at the same rate.
 
#232 ·
yes, you read it right, some of the early 2nd gens like mine have the 2.73 rearend. It even states that on the original window sticker in my glovebox. 2.73 mated up with the long gears of the NV3500 makes for some good highway mileage. i even have TBI injection too, and I still get 23-24 with A/C on
 
#235 ·
hey quick question!!!

i have an 82, but there is no stickers in my glove box...i know its all original, when did they start putting rpo stickers in the glovebox?

oh, and i found a gu4 g80 at the yard today, 75 bucks, i just dont feel like taking it out of the blazer. ill just go buy a real posi.
 
#238 ·
Since the stickers just have the codes all listed one after the other in rows and columns, how does one know where on the sticker to look? It's just a PIA sometimes trying to find a code for something when you have no idea where on the sticker you need to look or what the numbers could be.
 
#240 ·
Heh .... I have probably 100 pages of this stuff. If I could ever find enough time to scan it all, I might.

Here's a couple pages with a ton of axle codes. (click)

 
#243 · (Edited)
thought i'd share my find.. (unless I did it wrong).. both wheels off the ground.. spinning one wheel twice gave 3.7 turns of the driveshaft so...

for a 84 pick-up 4x4 2.8l 5speed i have a 3.73

at 65 in 5th I have 2000 rpm with 31' tires

PS anyone in the CT are have a spare axle.. my ring and pinion gear are busted.. :(
 
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