What Axle Do I Have? Identify It in Five Minutes Without Pulling the Cover

Start with the cover, because the cover is the answer

You do not need to remove anything. Get under the back of the truck with a light and look at the outline of the differential cover against the housing. Manufacturers did not standardize these, which turns out to be useful — each family has a silhouette you can learn in one sitting.

Dana 30. A rounded square with 10 bolts, roughly 9 inches wide. On a Jeep it is only ever the front axle — XJ Cherokee, TJ, JK, and the non-Rubicon JL all run a Dana 30 or its modern equivalent up front. If you are under the front of a Cherokee, you have a Dana 30. There is no version of an XJ that came with anything else.

Dana 35. A smooth oval, 10 bolts, about 10⅝ inches across. The classic description is a circle that has been mashed down from the top. The fill plug is a black plastic disc that snaps into the cover rather than threading in, and many housings carry a cast D35 marking on the tube or the back of the housing. Ring gear is 7.5625 inches.

Chrysler 8.25. This is the one people get wrong, because at a glance it is a dead ringer for a Dana 35: same 10 bolts, similar rounded outline, similar footprint. Run your hand along the bottom edge of the cover. The 8.25 has a flat spot roughly 3 to 4 inches wide at the bottom. The Dana 35 is round all the way around. That single check settles it every time, and it matters more than the badge on your fender does — XJ rear axle content is split between these two, and which one you got does not track neatly with year or trim.

Dana 44. An irregular hexagon — visible flats and corners rather than a smooth curve. Forum shorthand calls it muffin-shaped and that is about right. The fill plug is steel and threads into the cover, which is the fastest single check separating a 44 from a 35 if you are reaching blind. Ring gear is 8.5 inches, and the whole assembly looks meaningfully beefier once you have a 35 to compare it to.

The fill plug trick

If the cover is caked in mud and grease — which, if you are asking this question, it probably is — skip the outline and find the fill plug. Plastic, snap-in, pries out with a screwdriver: Dana 35. Steel, hex or square drive, threads out: Dana 44. It is a two-second check that survives a filthy undercarriage, and it works from an awkward angle where you cannot see the whole cover at once.

While you are down there, note whether there is a drain plug at all. Most of these axles have none — you change fluid by pulling the cover and resealing it, which is worth knowing before you buy a case of gear oil and go looking for a plug that was never there.

Modern Jeeps switched to metric names

If you are under a JL Wrangler or a JT Gladiator, the Dana 30 and Dana 44 names still get used casually, but the actual parts are AdvanTEK axles with metric designations. The number is the ring gear diameter in millimeters:

• M186 — front, the Dana 30 equivalent (Sport and Sahara)
• M200 — rear, the Dana 35 equivalent (Sport and Sahara)
• M210 — front, the Dana 44 equivalent (Rubicon)
• M220 — rear, the Dana 44 equivalent (Rubicon; optional on Sahara)

Two things people learn the expensive way here. First, an M220 is not a JK Dana 44 — gear sets, lockers, and axle shafts do not cross over between AdvanTEK and the older housings, no matter how similar the marketing name sounds. Second, the rear axle on a non-Rubicon JL depends on the build, not the trim badge: an automatic with an anti-spin differential can leave the factory with the M220 while an otherwise identical truck gets the M200. On 2018–2021 units there is a quick visual tell — the M220 puts its drain plug on the side of the housing, the M200 puts it on the bottom. If you want certainty, pull your build sheet by VIN.

Toyota and Ford, briefly

Toyota (4Runner, Tacoma). The 8-inch rear is the common denominator, but there are two of them. The base 8-inch uses a 2-pinion carrier; the 8-inch V6 uses a 4-pinion carrier and 29-spline shafts and is the stronger unit. Third-generation Tacomas with the factory electronic rear locker stepped up to an 8.75-inch ring gear with 34-spline shafts, which is a genuinely different axle from the open-diff 8-inch V6 in the truck parked next to it. If you are buying gears or a locker for a Tacoma or 4Runner, confirm which of the three you have before you order — this is the single most common Toyota parts mistake.

Ford (Bronco). Fewer variables here. Every sixth-generation Bronco runs an M220 rear with 32-spline shafts. The front is where the split lives: M190 on base and mid trims, M210 on Sasquatch-equipped and Badlands trucks, which also brings the front locker. If you are shopping a used Bronco specifically for the axle content, the front is what you are shopping for.

Reading the tag: BOM numbers and ratio tags

Dana has stamped identification tags onto its axles since the mid-1960s, held to the housing by two of the differential cover bolts. The number that matters is the Bill of Materials, or BOM. Pre-2004 it is six digits starting with a 6, followed by a dash number: 610604-2. From roughly mid-2004 onward Dana moved to seven digits starting with a 2. That BOM identifies the exact build — model, ratio, differential type, and every component the axle left the factory with. Any differential shop can decode it for you, and it is the most reliable answer available short of opening the housing.

Separate from the BOM, many axles carry a small ratio tag under a single cover bolt reading something like 3.73. An L suffix — 3.73L — means a limited-slip differential, which changes your fluid (you need friction modifier) and changes what you can install later. If the tag is present, believe it only if the axle is original. Tags get lost during cover replacements and they get left in place during re-gears by people who did not care, so a tag on a truck with an unknown history is a starting hypothesis, not proof.

Finding your gear ratio when there is no tag

You can measure the ratio directly. Both wheels on that axle need to be off the ground, which means this is a jack-stand job on level ground — a floor jack alone is not support, and nothing about counting driveshaft turns is worth getting under a truck held up by hydraulics. Chock the wheels still on the ground and confirm the stands are seated on structure before anything else.

With the transmission in neutral and the transfer case in 2WD, mark the driveshaft with chalk or tape and mark one tire. Rotate that tire two full revolutions and count driveshaft turns. Half that count is your ratio. Roughly 7½ driveshaft turns means 3.73; about 7 means 3.55; about 8¼ means 4.10; about 9½ means 4.56. If the axle has a locker or a limited slip that drives both wheels together, one tire revolution and the same count works. The two-revolution version exists because an open differential sends half the motion out the other side — if you spin one wheel and the opposite wheel turns backward, that is what you have.

The definitive method is to pull the cover and count teeth: ring gear teeth divided by pinion teeth is the ratio, exactly. A 41-tooth ring on an 11-tooth pinion is 3.73. 39 over 11 is 3.55. 41 over 10 is 4.10. 41 over 9 is 4.56. 39 over 8 is 4.88. Since you are draining the fluid to do it, plan this for when the axle is due for service anyway, and have a cover gasket or RTV and fresh gear oil on hand before you start.

Why the answer changes what you buy

This is not trivia. A Dana 35 and a Chrysler 8.25 take different fluid volumes, different covers, different gear sets, and different lockers, and the 8.25 in a 29-spline configuration is the stronger of the two for most builds — which means finding out you have one is often good news you did not know you had. The Dana 35 carries C-clip axle shafts, so a snapped shaft can let the wheel leave the vehicle; the same failure on a non-C-clip axle usually leaves the wheel attached. If you are planning 33s or larger, that distinction belongs in your decision, not in the footnotes.

The same logic runs through every platform on this list. Ordering 4.56 gears for a Tacoma 8-inch V6 when the truck actually has the 8.75-inch locker axle costs you a restocking fee at best. Buying JK Dana 44 parts for a JL M220 costs you the same. Five minutes with a flashlight, before the credit card comes out, is the cheapest step in the entire build.

The honest verdict

Cover shape gets you to the right family. The fill plug settles a 35 versus a 44. The flat bottom edge settles a 35 versus an 8.25. The tag under the cover bolt gives you a ratio you should verify if the truck has any history you cannot account for, and counting driveshaft turns confirms it without spending a dollar.

If you are still not certain after all of that — a swapped axle, an aftermarket cover, a housing with no legible markings left — pull the cover and count teeth. The number will not lie to you, and you will get a look at the gear pattern and the fluid condition while you are in there. Either way, you end up knowing exactly what you own, which is the whole point of doing this before you order parts instead of after.