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What stock gears do I have?

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dendeanda

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I have an 85 K5 with what i believe to have stock differentials and gearing. If they are stock, then they are GM Corporate 10-bolt (8.5-in. 1.28 Axle Dia. 28 Spline). What is the gearing on those? I wanna put some 35x12.50's on. Wanna know if thats too big for stock. I got a really strong '89 carbed Vortec 350 with an AirGap intake pushing 33's right now with a 700R4 and 208 tcase. Thanks guys!
 
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Glove box, sticker in there will have your "RPO's". Look for codes that start with "G" if three digit (like GU4) or codes that have the second character "G" if four digit (like 1GU4)

G80 is going to be the locker, but the other G code will be your axle ratio, IF it wasn't changed. *Any* 1985 K5 stock gearing is going to put a lot of strain on that 700 with 35's.
 
Well my glove box is ZipTied right now due to my lock handle falling out but I think i'll manage putting another one on after! So youre saying I should look for something like G373, which would be 3.73?

As far as the 35's, i should be more concerned about the 700 instead of the gearing itself?
 
No, the RPO will have no correlation visually to what you actually have. It will be something like "GU4" then just search the web for an RPO decoder. There are tons of them out there.

I'd be more concerned with the transmission. The highest numerically gears you'd have will be 3.73, I think most will say that with 35's and no transmission cooler, even those will generate a lot of heat for the transmission. Probably be pretty bad on acceleration too, but a lot of that WILL depend on your engine. Stronger engine, better acceleration as I'm sure you already know. :)
 
Here's a bunch of GM axle codes and years instaled.
GH2 76 87 AXLE,RR 2.29 RATIO.
GH3 86 88 AXLE,RR 2.77 RATIO.
GH4 76 81 AXLE,RR 2.28 RATIO
GL1 76 81 DIFFERENTIAL CARRIER,RA AXLE,ALUM
GM1 87 87 AXLE,RR 2.59 RATIO
GM3 85 88 AXLE,RR 3.45 RATIO
GM7 82 83 AXLE.RR 2.39 RATIO
GM8 76 88 AXLE,RR 2.56 RATIO
GN3 76 81 DRIVER CONTROL LEVELING
GS4 85 86 AXLE,RR 3.70 RATIO

GT4 76 88 AXLE,RR 3.73 RATIO
GT5 76 88 AXLE,RR 4.10 RATIO
GT8 76 88 AXLE,H.D. RR 4.10 RATIO
GU1 76 87 AXLE,RR 2.41 RATIO

GU2 76 88 AXLE,RR 2.73 RATIO
GU3 82 85 AXLE.AR 2.93 RATIO
GU4 76 88 AXLE,RR 3.08 RATIO
GU5 76 88 AXLE,RR 3.23 RATIO
GU6 76 88 AXLE,RR 3.42 RATIO
GV4 82 87 AXLE,RR 3.36 RATIO
GV8 82 85 AXLE,RR 2.72 RATIO
GW4 84 85 AXLE,RR 3.31 RATIO
GW6 85 88 AXLE,RR 3.27 RATIO
GW9 76 88 AXLE,RR 2.93 RATIO
GX3 82 88 AXLE,FRT 3.33 RATIO
GX4 76 83 AXLE,RR 3.75 RATIO

GX5 76 83 AXLE,RR 4.09 RATIO
GY5 76 87 AXLE,FRT 3.65 RATIO
G44 85 87 AXLE,RR 3.07 RATIO

With a 700R4. To keep from toasting it. You should be turning at least 1900 RPM In Od at 65 mph
 
Ok, lets take that first code for example: GH2 76 87 AXLE,RR 2.29 RATIO. Is the "76" the year or the "87", or both are years?
 
OK guys, I got some codes. WOW! I didnt know that little piece of paper was in the glove box, so much info. There are some codes that raise more questions. Here are some codes I got:

ORDER NO 9K5967- Im guessing the order number from the factory?

1G80 LOCKING DIFFERENTIAL- Does this mean I have lockers in my differentials? or just states that I have 4X4

1X6Z LOCKING HUBS AUTOMATIC- Which is strange because I have MANUAL locking hubs that I have to physically turn, so I'm not sure if the whole front diff/axle was changed out or just the hubs...

1GU4 3.08 RATIO- This is what the truck came with but I dont know if that has been changed out due to the swaping of my hubs.

I also found out that I have stock quad front shocks! SWEET! I've never seen another blazer with those around here.
K guys, any info would be appreciated! Thanks!

PS- I have a stock tranny cooler too which pertains my original issue.
 
G80 means that there is the government locker in the rear end. Relatively weak automatic locker, known to blow up when bigger tires are used. 10 bolt and 12 bolt are the weakest.
X6Z - These could have been changed out by any previous owner.
GU4 - 3.08 gear ratio could have been changed out, but that is not a guarantee. The only way to know if they have been changed is to pull the diff. cover and check the numbers on the ring gear, or count the teeth on the pinion and the ring gear. If it has not been changed, you won't want to run 35's. With 33's right now, acceleration is pretty slugish, and the engine really has to work when climbing hills or even driving down the freeway.
 
Another option:

  • Block the truck
  • Jack up one back tire
  • rotate the tire 2 revolutions
  • count how many times the driveshaft goes around - mark it with some chalk or something
  • if it goes around just over 3 times you have 3.08s,
  • almost 3 and a half times, you have 3.42s,
  • 3 and 3/4 times, you have 3.73,
  • 4 and a bit, 4.10s and so on.
  • unjack you truck
  • remove blocks
  • go for a drive (optional but recommended :wink1:)
 
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