Gear ratios

[Catman]

The decal in my glove box was scraped off when I got the K5. Is there any way to find out what ratio my stock 10 bolts have without opening up the differentials? It's a '90 K5. /forums/images/graemlins/k5.gif /forums/images/graemlins/usaflag.gif

 

[imiceman44]

There are 2 ways:
Guessing or turning the wheel and counting revolutions.
Guessing I woul say you have 3.42:1 it was the common ratio.
If you want to know with as close as you can get without opening the cover.
Lift one side, chalk a mark on the driveshaft, and a line on the tire or wheel. Turn the wheel one full turn while counting the driveshaft turns, and then multiply the number by 2.
If you have a locker then you have to lift both sides and then the number you get is the number.
If you had done a search, it's been asked many times in the past few months.
Please do searches first, all the questions have been answered in the past 2 years, and only custom setup questions are new, you can be sure that any stock truck related question has been extensivly exhausted.
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[BlazerGuy]

Check this out:
Calculators

 

[imiceman44]

[ QUOTE ]
Check this out:
Calculators

[/ QUOTE ]

I really tried, but couldn't find a way to use those calculators to get the gear ratio.
Did you really understand the question?

 

[dyeager535]

Believe in 1990 you only had two or three options, and if three, I'm sure they were 3.08, 3.42, and 3.73.

Well, I *could* be mistaken...check this: http://brochures.slosh.com/1990/gmc8.jpg

Since that is all "R/V" series trucks for 1990, I know for a fact some of those ratios were not available for the Blazer/Jimmy. Might add 2.73 as another possibility on yours, but I think the 4.10/4.56 could NOT be had in a 1990 K5. Some of that has to do with destination, if you look back at earlier K5 brochures, you will see that some ratios were required with certain engines or transmissions, and some combo's were not allowed in CA at all. Lots of variables.

BTW, on RWD cars, there was a 2 or 3 digit axle code stamped on the passenger side rear axle tube, about halfway between the diff and wheel, which when decoded would tell you what the axle ratio was, and whether it was an open or limited slip (or locker) diff. Not sure if that continued into trucks or not.

 

[91GMCSuburban]

I was told there is a code on each axle housing that tells you what gear ratio was installed. Check with your local gear shop for location and meaning.

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[Catman]

Thanks for the info. The "brochure" seems to suggest 3.73 with the tow package. A mite to high for the 35's I'm about to put on. By the way, I did do a search. I'll do the chalk test this weekend if all the beer in the world disappears! /forums/images/graemlins/thumb.gif /forums/images/graemlins/k5.gif /forums/images/graemlins/usaflag.gif

 

[88sub4x4]

There is a code on the top of the right axle tube about 2 inches from the gear housing. I had to wire brush mine really good to find/read it. I'm not sure if it is the same as the RPO code though. Some junkyards have a book that deciphers it. /forums/images/graemlins/burb.gif

 

[dyeager535]

Not the same as the RPO code, and I've not seen any comprehensive listing for them anywhere either.

The breakdown's gotta be in either the service manual for the correct year, or the parts manual for the correct year.