Do not trust internet differential calculators

[Hedgehog]

Really, they just might make you feel foolish.

This weekend I ran all the numbers an accurate speed, the RPMs from a good tach, and the height of a new set of tires. I took several readings at different speeds. When they were ran through ne of the many differential calculators on the web they came out to 4.085 to 4.098. Wouldn't you agree, for all practical purposes the gears should be 4.10's.

So, tonight I pulled the front cover, sure enough the gears are 3.73's . :down: Hey it's gotta be right I saw it on the internet.......

No mater what, I'm glad the cover was pulled. When removing the bolts on the bottom of the diff. nice clear water dribbled out. It was obvious what was coming next. A chocolate milk shake mess. The gears look good though. I'm still going to go with more of a road gear though. I think the carriers are good for those gears.

 

[billyballer]

Another easy way to check differential gear ratios with an open diff is to jack up one wheel, mark both the driveshaft and the tire with a paint pen for reference, spin the wheel two full rotations and count how many times the driveshaft spins. 3 1/2 times would be about 3.55:1 etc. There is probably a way to do this with a locked diff but I'm not 100% certain on that one.
Nothing like counting teeth to be 100% accurate though, if you don't mind popping the diff cover.

 

[Hedgehog]

Thanks Billy, that method has been posted every time I have written about this and it's the only method I haven't actually done. While it seems easy enough, the numbers seemed to be all over the place so I didn't trust the accuracy of the method. In hind site it probably would have served me better.

The front differential was the only part I haven't done a service on, so no matter what, it was time to do an inspection. Really, if you have a decent oil catch pan, pulling the covers isn't messy or difficult. The hard part is cleaning the old gasket material off. With an angle grinder and wire wheel attached even that isn't difficult.

 

[CJ]

Just in case anyone else sees this post, check out -->> How to tell what gear ratio is in your CJ. - Jeep-CJ Tech. Articles

 

[hole]

Hedgehog, When you put it in 4wd is there any binding or bucking? I'm playing devils advocate here, but you might still have 4:10's in the rear and 3:73's up front. Not saying the all mighty internet has any false info on it but PO's do funny things to CJ's .You've done so much to your rig already that I hope for your sake it's the internets fault

 

[Hedgehog]

On the 17th we took a trip and logged ~50 miles in 4wd. Some was on a very hard dirt road, some was over rocks and some in soft sand, all the while, at least at first, I paid a lot of attention to how the jeep ran, tracked and drove. It never complained about much of anything, driving in 2 and 4 wd felt essentially the same (it was obviously truly in 4wd, no gear problems) so I would be shocked to find different gearing in the rear. Remember this comes from someone old enough to remember when some rigs were slightly positively driven with the front wheels being driven a few % points faster than the rear.

CJ - I hadn't seen that thread. Usefull information there. It seems to have stuck in peoples minds because a variation of the information given is posted almost everywhere the question of gear ratio's is asked.

 

[mstar]

The difference that you referred to on the R&P ratio is like I/100.As I recall it's like 4.89:1 in front and 4.88:1 in rear.Also the miscalculation is less then 10% which is not much considering the data you used isn't exact science. Actually I believe the calculator is designed to give a aprox. RPM number for a given ratio and tire size. Yes it works both ways but not as effective. Counting teeth as you did is best. mike

 

[Hedgehog]

shame - I didn't even count teeth ..... all the information was stamped on the ring gear. birthday (9/16/74), manufacturer (DANA), serial number (?), gear count (41/11), Gear ratio (3.73), 889 (?) ...... I've got it all written down at home

 

[BusaDave9]

You can never use those charts to find out what gears you have. The reason is the tires. They are never exactly the size they say they are. There are too many factors such as flex, air pressure, bias ply radials. Even when everything else is constant tires from different manufacturers vary greatly in size.

 

[Hedgehog]

Yeah I did consider actually measuring my tires, granted it would be a cold, standing still measurement, but probably closer than the 10.5x31x15 stamped on the tire. But to my defence I did run a bunch of numbers including varying the RPM's by a few hundred up and down, then the speed, I never messed with the tire size though. Nothing I tried seemed to change the result much though.

 

[007]

I don't know, as mstar says, you were only off 10% which isn't bad. I've always used Gear Ratio Calculator to run calcs on this as you can input the exact diameter of your tire.

 

[Petescj]

No mater what, I'm glad the cover was pulled. When removing the bolts on the bottom of the diff. nice clear water dribbled out. It was obvious what was coming next. A chocolate milk shake mess. The gears look good though. I'm still going to go with more of a road gear though. I think the carriers are good for those gears.

This is why Im glad you took the time to do it right. I changed every fluid in Vickis Laredo when I got it but didnt get around to the Dana 300 . That Transfer Case is now awaiting a rebuild. When I open it up I'll see what happened but I wish I wouldve changed it out. Hindsight. I wish you had a Dana 30 AMC20 combo. Idve shipped you my carriers and gears to trade for sure. My Limited has the small carriers. Not sure if its 2.72 or 3.31.

 

[Hedgehog]

Petescj - No matter what I was going to break the seal on the front. The PO must have either ran quite a bit in the water or the Jeep sat in a puddle long enough to defeat the seals. I posted on this before, the front hubs were full of rust and the grease was well mixed with water. There's not much damage, possibly some damage under the gaskets.

I'm still a little bit in the woods, the 30 would need to be changed the break point for the Dana 30 being 3.73 goes up with the 3.54 going down. The Dana 44 's break point is 3.73 goes down and the 3.9? goes up. If your looking for the bigger or taller carrier for your Dana 30 we just might have a chance to work something out.

 

[Old Dog]

You can never use those charts to find out what gears you have. The reason is the tires. They are never exactly the size they say they are. There are too many factors such as flex, air pressure, bias ply radials. Even when everything else is constant tires from different manufacturers vary greatly in size.

You can never go by what it says on the tire. Most of the time they are at least 1" shorter than what they say.
Park on some concrete or blacktop and measure from the ground to the center of the wheel (axle) and times it by 2. (you only drive on the bottom)
Measuring the tire off the jeep does no good.
When I plug 31" tires (what mine really are, 15.5" to center) in a cal. instead of 33" like my tires say, it comes out really close to the right rpm.

 

[Hedgehog]

Excellent point on measuring from the bottom to the center, then x2. I would have measured sitting on pavement to the top thinking that I'd taken are of the squish area when in reality it was only 1/2.

 

[mstar]

All you have to do is jack up one wheel and measure the circumference of the tire if you really want to cut down the error factor. just eliminates the dia. x pie = equation and much more accurate.Having a digital tach would be a big plus also. I think you said mph was measured by GPS. that is the most accurate method that I know of. mike

 

[BusaDave9]

Measure the radius of the tire from the center to the ground?
Measure the circumference around the tire?

Here is why I think the tire is the main reason these rear ratio charts are inaccurate. There is always tire flex and it's hard to tell how big a tire really is (at least from a rolling and gearing perspective). Imagine a tire aired way down. The axle is now low to the ground but if you look at the tracks (tread) it still takes almost as far to rotate one revolution.
My opinion is that the best method is to see how far you travel when the tire rotates once and determine the circumference from that.

Okay, okay, I'm splitting hairs but my point is the tire is the factor that if very hard to tell just how big it is. You'd think the size of a tire would be irrefutable but it's not.

 

[mstar]

Well I think everyone would concur that the tire diameter is the only real variable in the equation. Seems that maybe the use of the calculator is not what some are looking for. I don't think it was ever intented to be exact science. If you can't divide the circumference of the tire into one mile and come up with a pretty close number then I guess it's no good.mike

 

[BusaDave9]

Seems that maybe the use of the calculator is not what some are looking for. I don't think it was ever intented to be exact science.

Those charts are not an exact science if you plan on changing tire sizes. They also don't work if you are trying to figure out what gearing your axles have.
But the main use for them is changing the axle gears. In that case you must keep the same tires on for there to be any trust in these gear charts.
If you go from 3.73 gears to 4.88 gears then your RPM and gearing will change by 30.83%. No if's, and's or but's about it as long as you keep the same tires on your jeep. (does that count as an "if")

 

[Hedgehog]

Well, this has gone farther than I expected and I (we) did learn something. Thank you all for your input.