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ggevaert 07-24-2011 10:28 AM

Rear End Ratio - measured and number looks way too low - what am I doing wrong?
Hello Everyone,

Finally got a chance to try to determine my rear-end ratio (1984 GMC pickup truck with stock rear end sofar) so followed the steps whereby I jacked up the truck and marked the driveshaft and tire then spun the tire one circle to determine the number of revolutions the driveshaft makes.

And; I get 1 and 1/3 turns out of the driveshaft for one wheel rotation. Just to be sure I spun the driveshaft by hand one rotation and the tire went 3/4 of a turn.

I was expecting something like 2.73's (as per data sheets for that year) but 1.3??

Am I doing something wrong or is that possible. The instructions don't say anything about doubling the number you come up with (which would then make more sense)

Thank You :welcome:

joe_padavano 07-24-2011 10:50 AM


Originally Posted by ggevaert
The instructions don't say anything about doubling the number you come up with (which would then make more sense)

Doubling is EXACTLY what you need to do for a non-posi axle. Since you're only spinning one wheel, you're losing 2:1 in the spider gears. Now your 2 2/3 looks a lot like 2.73:1. By the way, I usually turn the drive shaft ten times. It's a lot easier to see the difference between 32.3 and 34.2 turns of the wheel than 3.23 and 3.42, for example.

sqzbox 07-24-2011 11:52 AM

Yoy could probably do better just posting your tranny and RPM at 60 mph on here to find what ratio it is. :D

ggevaert 07-24-2011 07:08 PM

Thanks, that makes sense then and matches what the spec sheet says. I did not know that about the doubling but makes perfect sense.
I wanted to double check as the person I got the truck off of did not indicate whether it was changed and if it was already between a 3.40 and 3.80 then I wasn't going to bother as that matches the engine/tranny setup. However, I forgot about the posi part so even if the gears were correct I would have needed to change it anyway.


NEW INTERIORS 07-24-2011 07:11 PM

You have to turn both tires together.. :thumbup:

fast68 07-25-2011 10:06 PM

2.6 is what you found.

thats basically gonna point at 2.73...

sometimes when youre counting rotations you end up needing to double what you find, depending on how you did it

one wheel on ground or both off ground.

will change what number rotations you get.

doing it with one wheel on the ground will be most acurate and easiest...

note: 2.21 and 2.56 were available and uncommon ratios in these trucks as well over the years, after 1972 when the awesome CST generation of trucks ended.....

not as common ratios as others such as the 2.73 and 3.07 and such....

GM had alot fo oddball ratios in the trucks....

2.56 and 3.40 were a couple of the least common ones....

2.21 was another uncommon truck ratio too actually.. i forgot about that one almost... ive only seen one or two actual 2.21 trucks ever in my life, listed on the RPO sticker and all...

ggevaert 07-27-2011 07:37 PM

Also likes Chevy trucks and boobies - go figure
Thanks Fast68, guess what, I also like Chevy's and Boobies, weird eh! :)

I actually had both wheels off the ground when I did it and turned the passenger one and measured the driveshaft. Then I went and also did the measuring by turning the driveshaft and it spun the passenger tire only (no Posi).

Thanks Everyone :welcome:


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