now I'm really confused

[brother rat]

The axle code on my door sticker is 86. That should be a 7.5" open 3.73. What's actually under the truck is an 8.8" (original owner must have swapped) and the axle tag indicates open 4.11 gears. When I lifted up the rear end this weekend, for every complete tire rotation the driveshaft does ~1.8 rotations. So, now my mind is boggled. The truck is not equipped with a tach to confirm RPM at a given speed.
What gives?

 

[PlumCrazy]

To check the ratio both tires need to be turned at the same time, or else the diff will skew your results if the other wheel isn't turning as fast.

-PlumCrazy

 

[Captain Ledd]

28 spline axles: 2.80" diameter tubes (8.8" circumference)

31 spline axles: 3.25" diameter tubes (10.2" circumference)


*edit: Just re-read your original post. Ignore the next part, but if you're really curious you can still do it. The tube method above will tell you what spline carrier you need for the 8.8".

At this point I'd suggest pulling the cover. if your pan is clean and you clean up around the cover to keep crap from falling in it you can re-use the fluid.

There will be markings on the ring gear: the ratio, teeth on the pinion, teeth on the ring, and the ring gear diameter, along with some part numbers.

 

[brother rat]

Thank you both for quick responses.

the diff will skew your results if the other wheel isn't turning as fast.

that's what I was thinking.

To check the ratio both tires need to be turned at the same time

This might prove difficult.

At this point I'd suggest pulling the cover.
There will be markings on the ring gear: the ratio, teeth on the pinion, teeth on the ring, and the ring gear diameter, along with some part numbers.

This might be easier.

 

[4x4junkie]

When I lifted up the rear end this weekend, for every complete tire rotation the driveshaft does ~1.8 rotations.

Easy way to check ratio is jack up one tire and give it TWO complete rotations while counting the driveshaft turns (this because the differential spider gears are also turning which halves the driveshaft speed).