Gear ratio calculator

So I'm trying to find out a ratio to an axle without popping the cover off. What figures I do have come out to 3.73s I think, but please check my work here. @ 25 mph the engine is at 3900rpm with an ax15 3.83:1 first gear, and 32" tires.

It really is right in the middle between 4.10 and 3.73. If I could drive his jeep I think I'd be able to tell, but he's having some problems and I don't want to be the one it messes up on.

This is a dana 44, and If it's 4.10s I'm just going to straight trade him axles.

You say 32" tires, but what is the actual diameter of the tires? Most listed diameters are not the true diameters, especially accounting for wear. It will make a difference in the calculation.

Why not just pull the cover to be sure?

Why not jack up one rear wheel, rotate it one full turn, and count the driveshaft revs? Divide by two, and viola, your ratio.

there is a 4.07 ratio in the 44 too. Rare, but out there.

Ok If I change that for wear I'd call it an even 31" and counting revolutions of one tire doesn't work because you have to take into account the spider gears, It has a tag that says limited slip on it, so they must be stock because who is goofy enough to put the tag back on? Unfortunately this axle is not stock where it is. It's in a 90'.

So when I change the tire figure to 31's I'm getting even closer to 3.73. This must be the case.

you can pull the diff cover and there will be numbers on the side of the ring gear, like 44 9. that is the number of teeth on the ring gear and pinion gear. divide the bigger number by the smaller number and that gives you the ratio. for example, 44 9 would be 4.88's. otherwise, jack up the whole axle and rotate the driveshaft and count the wheel revs.


edit just noticed you dont want to pull the cover so ignore that.

Fully aware of how to calculate all of the necessary things. Just don't want to dive into somebody's diff cover if I don't have to. I told him I'd only be willing to do the trade if his was the same gear ratio as I'm not quite ready to pony up the cash to go to 4.88s PLUS I'd have to buy and have installed 3.73s into my c8.25 to do the trade.

I'll just get a junkyard dana 44 and build it up slowly from scratch as this trading axle thing will just cost way more than necessary. I'll just end up selling my 8.25 to one of you poor d35 saps for some outrageous price to recoup some of my build.

The problem with figuring axle ratio by engine RPM @ X-mph and wheel size is that the stock speedo can be out as much as 10% (or more) stock. My stock speedo was out 10% from the factory, read 110kph at 100kph. Course, that makes for far fewer speeding tickets, and made 'feel' like I was goin' fast.
I have down to a 5% error now with 4.56 gears, 32" tires and a speedo sensor regear.

Then add larger than stock tires and the question of was the speedo re-calibrated ...

So, it's either spin the driveshaft or pull the cover to be sure really.

does it look like a stock XJ D44, or more like a TJ D44 with spring perches added? The XJ version will be more likely to be 4.10, whereas the TJ version came in both 3.73 (stock base) and 4.10 (option) varieties.

Without pulling the cover and counting teeth (OR finding a handy stamp on the drive gear), or a tag on the Diff...

Spinning the DS and counting the number of revolutions to get one revolution from the wheels is the best estimator of gear ratio.

Rather simple really. Jack up the rear of the vehicle (if axle is in the vehicle) so both tires can free spin. Take one tire off an use that hub as a reference.
Mark the hub.
Mark the pinion.
Turn the pinion until the Hub has made 1 revolution. A 3.73 ratio will be 3.73 turns of the pinion to 1 of the axle (DUH, right )

Using tire diameter and RPM, gets you your "Effective" Gear Ratio, not your axle's actual gear ratio.

WB9YZU said:

Without pulling the cover and counting teeth (OR finding a handy stamp on the drive gear), or a tag on the Diff...

Spinning the DS and counting the number of revolutions to get one revolution from the wheels is the best estimator of gear ratio.

Rather simple really. Jack up the rear of the vehicle (if axle is in the vehicle) so both tires can free spin. Take one tire off an use that hub as a reference.
Mark the hub.
Mark the pinion.
Turn the pinion until the Hub has made 1 revolution. A 3.73 ratio will be 3.73 turns of the pinion to 1 of the axle (DUH, right )

Using tire diameter and RPM, gets you your "Effective" Gear Ratio, not your axle's actual gear ratio.

Click to expand...

What he said. You cannot base gear ratio calculation on RPM and tire size, as there are so many variables that the margin of error is beyond the gap between common ratios.

Don't worry about the spiders, they will not cause an issue with this test. Also, it was said 4.07 exists...even if this is the case, it will work fine with 4.10 or 4.11, so no worries. Hope it is what you are looking for. :wave:

The only way spiders won't interfere is if both tires spin equally. If you just spin one wheel the ratio would be doubled.

Hasta

Redxj5150 said:

The only way spiders won't interfere is if both tires spin equally. If you just spin one wheel the ratio would be doubled.

Hasta

Click to expand...

Whoops...meant to quote the other guy as well that said to divide by two. :greensmok