How to tell gear ratio by spinning tire

I've done this before but can't remember how. I put in a front axle from a junkyard, which they told me was a 4.10 axle. I've got it all hooked up, and put it in 4Lo, going straight, just for about 10 feet or so. It felt like the rear tires were spinning more than the front ones, and the rear ones even chirped (on pavement with loose gravel). So, I'm comparing my old axle (3.07's) to this one (4.10's). Here's what I have. For every revolution of the tire or rotor, the 3.07 axle turns the yoke 1.5 turns and the supposed 4.10 axle turns the yoke 1.75 turns. Can somebody explain the math behind all of it and how to tell without counting ring gear teeth? I checked the percentage differences and it leads me to believe that the new axle has 3.55's in it, but I'm not sure. TIA.

I only jacked one tire off of the ground, not both. Would that account for getting the values above? Here's some information I found on it:

"Counting driveshaft rotations
This is probably the easiest way to measure your rearend ratio, as it doesn't require removing the differential cover. Jack up the rear end of the truck (so that both rear tires are off the ground) and make a mark (or pick a spot) on both the driveshaft and a rear tire. With the transmission in neutral, spin the tire by hand through one rotation, and count the number of times the driveshaft turns. The number of driveshaft rotations is your rearend ratio. (For example, if the driveshaft turns about three and a half times, the ratio is probably 3.55. If its almost three and a quarter, the ratio is probably 3.21. Almost four times, 3.91s, etc.

Note: The procedure above applies if you have a Sure Grip (limited slip) differential or a locker. If you have an open differential, the procedure is the same, but you will need to spin the tire through two rotations instead of just one. If you do not know what type of differential you have, see elsewhere in the DML FAQ to learn how to determine this.

If you do not have a jack, you can also do this test on a level area by rolling the car enough to spin the tire once. Obviously, be extra careful if you do it this way (get a couple of buddies to help you) - jacking up the rear is safer."

It sounds like you might have 3.55 gears in that thing.

Why don't you .......

just pull the cover and look at the numbers on the ring gear? i have 4 Dana axles in my shop and every one of them has the ratio stamped into the ring gear. Its the tooth count actually that is stamped in there.

Yep, I've got 3.55's in it. Shame on me for not checking before goingth rough the hassle of putting it in and shame on them for selling me the wrong axle. Here we go again.....

With only one tire off the ground, the differential is doing what differentials do -- so with an open rear (oops -- front) rotating only one wheel only turns the driveshaft half as many revolutions as if you could rotate both wheels at the same time.

Make certain you're rotating the tire exactly one revolution, and that you count drive shaft revolutions accurately, and then just double what ever you count to get the actual axle ration. Half of 3.07 whould be very close to 1.5, and half of 3.55 would be close to 1.75.

If the axle were a 4.10 diff, you should be getting 2+ revolutions on the drive shaft.

OK, here's another twist to this question. How hard is it to install your own gears? Obviously, I've never done it before. What are the possible hazards of doing it wrong, other than it eating up the ring or pinion? Will it be dangerous or lock up the axle in the middle of driving somewhere? Do you need any special tools? If I were to do this, I'd get a manual and read up on it, just toying with the idea.

Besides the knowledge and experiance youll need......

Press
Bearing puller
dial indicator
micrometer
an inch/pound torque wrench
a foot/pound torque wrench

and alot of time and patience.

xj92 said:

Will it be dangerous or lock up the axle in the middle of driving somewhere?

Click to expand...

While I'm all for learning how to 'do' stuff with my Jeep, it cost me well over $1000 to fix my "Previous Owner Learning Project" CJ5 Dana44 when it let go. PO taught himself to do quite a few things on that Jeep, one of which was setting up his own gears. The Trac-Lok, ring, pinion, and carrier all had to be replaced after it scrambled itself on the road. Luckily I was only moving about 10-15 mph, but had I been going faster that could have been quite a nasty scare.

If you do plan to do it yourself, see if there's someone knowledgeable in your area that you can have check your work as you go.

Something else to think about,if you have 3.55's in this axle then youll will still need a new carrier if your wanting to go to 4.10's.The carrier break is 3.73-up/3.55-dn .

BTW:as I told BillR recently,Ive got all the tools(I do my own work),youre welcome to them but I "do" not do gears for anyone!

If you do want to try it yourself, make sure you give yourself plenty of time to do it. I just did both ends on my XJ for the first (and probably last) time. Luckily, I took off a week from work as I needed almost all that time. Took me 4 days, 10 hours each day to finish it. It is no fun at all. Unless you pull the axle, count on spending a lot of time laying on the floor. I must have removed and replaced the carrier 20 times per axle to get it right. Well, mostly right, my frontend still has a little more whine when in 4WD than before the re-gear even though it shows a good pattern on the gears. I wish I was as lucky as some of those here and the new gears required very little adjustment. I'd imagine if I did it again I could do it in about half the time but still, it sucked.
I'll tell ya my story. When I finished up the D35 for the first time, got a good pattern, went for a drive, all sounded good and felt good. Got back home all smiles, pulled forward to back the Jeep in the garage, put it in reverse and hear a groaning/grinding sound while going backwards. Ahh Sh#*!!!! Had to tear it back down and basically start over.
Don't want to scare you off but just be warned.

Setting up gears can be costly if you don't do it right, also it consumes a lot of time if you do it first time. Better let a shop do it for you, i know a lot of guys that do anything on their Jeeps except gear setup.

I think I'll pass on doing the gears myself. The junkyard will refund the money for the axle since they screwed up, and I'll just look for another D30 HP with 4.10's already in. That seems to be the cheapest route from what I've seen. Thanks for all the info.

Should be able to find a D30 HP in 4-cyl XJs. (and TJs?) It's just a matter of finding a 4-cyl...

Jim