How does one PROVE the axle ratio of a used truck?

[DieselDrax]

Originally Posted by aircommuter

Probably because 2 turns give less chance for error.

When jacking one tire and checking with an open diff you need to count driveshaft rotations for 2 rotations of the tire. Why? When only one tire is turning it turns twice as fast for the same number of driveshaft rotations. When both sides of a differential are rotating then one rotation of the tires is needed.

Meaning if you have 3.73 gears and only rotate one tire one time you will only see 1.865 driveshaft rotations instead of 3.73 rotations.

[DieselDrax]

Originally Posted by TheHog

I am not 100% sure on Dodge products. But on GM vehicles there is a list of all the equipment the truck has in the glove box. There will be many codes. Just Google factory rear axle codes for your year, make, and model. Then match up the code.
GM axle codes are GU and a number. Hope this helps.

Dodge/Ram has a sticker in the glovebox listing the ratio, this should match the ratio the OP found when going to the Dodge self-service portal, punching in the VIN, and getting the factory equipment listing to find out what the truck came with.

[Goat Man]

Look in the glove box. There is a yellow sticker in there that givs the info. You can compare it but go with the build sheet.

[Sach]

Originally Posted by nick1236

There should be a tag on the axle that says the ratio, or gives you a BOM number (bill of materials) that you can look up.

Any dealer should also be able to decode axle tag too.

[DieselDrax]

Originally Posted by Goat Man

Look in the glove box. There is a yellow sticker in there that givs the info. You can compare it but go with the build sheet.

Ah yes, I totally forgot my 2500 had that! I had always been in the habit of pulling the online build sheet instead for my other Chrysler vehicles that it slipped my mind that my truck had it, thanks. That sticker should definitely match the build sheet.

[cmysstailights]

I have a silverado. In my glove box is my rpo codes (codes that list every option the truck was ordered with). Look for that then find rams rpo code list

[DougW]

Looking up the VIN, looking at a build sheet and checking there axel code will tell you what it was when it left the factory.

Keeping in mind the OP's question is how to PROVE the ratio, I have seen two answers that are correct. Either open the rear end and count the teeth or jack it up and turn the wheel while counting the revolutions of the drive shaft.

[TitanMike]

Originally Posted by DieselDrax

Why? When only one tire is turning it turns twice as fast for the same number of driveshaft rotations.

Some people learn this the hard way when they get stuck in the snow. One wheel is spinning like the dickens, the speedometer is reading 80 Mph, and suddenly the spinning tire explodes.

[rlh1957]

Tag on axle... but if axle gears failed and replaced with a different ratio, hard to tell.

[mj0528]

My vin shows my f150 to be an XL but I found a Ford employee provided the window sticker. XLT

[RamblerGuy]

Originally Posted by cfiowa

What you could do is jack the rear end up on the truck, make a mark on the drive shaft and a mark on one rear tire.
On the tire, rotate the tire 1 full revolution and count how many times the drive shaft turns past the mark you made on the shaft, this is your axel ratio.
For example. 1 full rotation of one rear tire equals 3.73 turns of the drive shaft.

You may have to put the auto trans in neutral when you do this so the drive shaft turns.

When using this method, be sure that the tire on the other side rotates exactly the same as the tire you're rotating. Maybe mark both tires and if possible have someone watch it. If both don't turn the same, hold one at a full rotation and move the other one to a full rotation.

If it has a limited slip differential, the differential will make the opposite tire turn with the one you're turning. If it's an "open" differential, the opposite tire may or may not turn the same, depending on brake drag, bearing drag due to cold lubricant, etc.

[Hersbird]

I bet the dealer is just using a generic Ram Truck description from whatever marketing program writes their ads. I have seen this a bunch. I would go by what the manufacturer said. Also there is a way to look up the original window sticker on some of these newer trucks. That will show exactly what was added as options. Of course any axle can be regeared and the factory, dealer, even the tag on the axle wouldn't know what was installed. I personally like to run it up to 60mph, with overdrive locked out, make sure the converter locks up, note the rpm at that point, and tire size, then plug it in to a gear ratio caculator.

Ps I just checked that window sticker program and it has to be 2013 up Chrysler, Dodge, Jeep, or Ram

[jaymz]

The only way to be 100% sure is to pull the diff cover and count the teeth on the ring and pinion and do the math, or find where the ratio is stamped on the ring gear.

Typically, the VIN will tell you what’s there, and normally I’d trust it more than the tech at the dealership. But with it being a used vehicle, there’s no way to know what previous owners may have done.

[jonzer12]

Lots of OEM ring gears have the ratio stamped on the circumference. You can pull the fill plug on the diff cover and use a flash light first in at attempt to avoid pulling the cover. Marking the pinion and counting revolutions doesn't work so good when you have a ford for example that could be 3.31, 3.55, or 3.73. Ratios are too close to be sure. Worked good in the old days when you had say 3.07, 3.73 and 4.10.