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73-87 Chevy _ GMC Trucks => Engine/Drivetrain => Rear Drivelines, Drive Axles and Gearing => Topic started by: 79BONANZA on August 15, 2002, 04:38:00 AM
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I have jacked both of my back tires off of the ground and turned my drive tire one rotation, my driveshaft only turns 1 3/4 turns. What is my gear ratio? The truck is a 79 Bonanza with a th400 tranny and a np205 transfer case. Only had it a few months and trying to get to know it better. Wouldn't know that much if it wasn't for you guys (read every post here). Thanks for the help, and the help you've given others
Jim in Texas
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that would make it a 1.75:1 ratio which as far as I know does not exist??? The highest gearing offered in 1979 was 2.76:1 (1/2 ton only). The lowest was 4.56:1 (one ton only).
Mike 
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This may be the obvious...but have you checked the inside of the glove box for your options sheet (build sheet...whatever)? Mine is a '74 and luckily it has survived so I know all the factory options that came on my truck. Like 83Chevy said, 1 3/4 turn just doesn't make sense. Did you mark your driveshaft to make sure you were counting properly? I know that may sound obvious too, but once that thing starts spinning it's hard to keep a visual reference. I don't know if it matters whether or not you have an open differential either...maybe try turning the other tire and see if the result is different??? Anyone else know if that makes a difference? Last ditch, pull the rear cover and check the gears visually.
Jason Harness
1974 GMC C2500 454/THM400/4.10
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Well, based on your truck, I'm going to go out on a limb and say taht you probably have either 3.73's or 4.10's with the 4.10's being more likely. Did you check the front and rear axles for a gear tag. I found one on my truck attached to a differential cover bolt on the front axle.
1973 K20 - 350/350/205
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Just looking at some old posts and it said to do the "tire spin" method you have to go two turns if your differential is open. This might make more sense for you...instead of 1 3/4 it's more like 3 1/2 turns...possibly indicating 3.56 or 3.73 gears. Hope this helps...
Jason Harness
1974 GMC C2500 454/THM400/4.10
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Thanks for the info guys. I do have a sheet on the inside of my glove box, but I talked to the previous owner of the truck and the guy he bought it from 7 years ago said the gears were changed, of course no one can remember what they were changed to
Guess I'll do it the hard way and pull the cover, just something fundamentally wrong with tearing apart something that doesn't leak for curiosity's sake. I'll do it in a couple of days (gotta have it to put in french doors) and let you know what it was.
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i have a 85K10 with a 350,th700r4,np208, and the build sheet says standard. What ratio is that?
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on my 86 the gear ratio is listed in te glovebox you just gotta look real hard to find it
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They're 308's, and no leaks. Thanks ya'll.
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Quote:
i have a 85K10 with a 350,th700r4,np208, and the build sheet says standard. What ratio is that?
3.08:1