Category Archives: Axle

Identify your axle gear ratio

Have an axle with an unknown ratio that you’d like to identify? Want accurate results? Here’s a simple, dead accurate method that will give you results on any rear wheel drive axle (or any front axle for a four wheel drive), as long as the axle in question is driven by a driveshaft. Sorry, front wheel drive folks, you’re generally out of luck on this one. This is a single-person technique. Helpers are not required, though one does make the counting a bit quicker.

Tools required:
Chalk, crayon, or paint pen
Jack stands

I make the general assumption that you already know to follow all the necessary safety techniques. If you don’t already know how to do anything required here safely, find a friend to learn from, or another source of knowledge. General work safety practices for cars abound on the internet. Hence, work at your own risk.


1. Determine if you have an open differential, or a traction aid (limited slip, locker, etc.). If you don’t already know, here’s how. Jack up the axle and put both sides on stands, so both wheels are off the ground. Leave the transmission in park or in gear (manual). Rotate one wheel by hand, while watching the opposite wheel. If the other wheel rotates easily in the opposite direction, you have an open differential. If there is resistance to rotation or no rotation, you have a LSD or locker. To verify a traction aid, place the transmission in neutral, and both wheels should rotate together when you turn one by hand. I’ll note differences in later steps between open and traction aid techniques.

2. LSD/Locker: leave the car on the stands. Open diff: lower one tire to the ground, leave the other on a stand.

3. Mark the tire and driveshaft. Put one mark on the tire sidewall where you can easily see it, and another on the driveshaft near the axle. You’ll need to be able to see both from where you’re working unless you have a helper, so they should both start out facing you. If you have a lot of tire clearance to your fenders, it’s often easiest to start with the marked spot toward the ground, and a block of wood or rock to use as a reference point. You want to be able to count tire turns to within a few inches of your starting point; more accurate than that won’t really be necessary.

4. Now, you’ll need to rotate the tire while counting turns of the driveshaft using your mark. Rotating forward or backward doesn’t matter. Turn the tire 10 turns for an LSD/Locker, 20 turns for an open differential. When finishing, try to count your last driveshaft turn to the nearest 1/4 revolution.

5. Divide the number of driveshaft turns you counted by 10 (same for both LSD/Locker and open). That’s your gear ratio.


  • You determine your pickup has an open diff, so lower one tire to the ground. You turn the tire 20 turns, and count 35-1/2 turns of the driveshaft. 35.5 / 10 is 3.55 – a common stock gear ratio for Ford pickups.
  • Your car has a limited slip, so you leave both tires up. You turn the tires 10 times and count about 27-1/4 to 27-1/2 turns. This will be either 2.73 or 2.75 depending on your axle make and what ratios are available.

The last number is always a bit of a fudge with this method, but always close enough, as you’ll basically never encounter a single axle make that has two different ratios available that are so close. You can always count on the first two numbers, such as 2.7x and 3.5x, being dead accurate, and that’s always close enough to identify the exact ratio once you know what axle family you have. For instance, with a Ford, the 2.7x example is going to be 2.73 if you have an 8.8″ or 7.5″ axle, or 2.75 if you have an 8″ or 9″ axle.

I’ve used this method many times over the years, since it’s dead on and works great whether the vehicle still has its stock gears or not; axle tags are only useful if no other previous owner decided to regear. I also take a crayon with me any time I’m headed to a junkyard or to purchase an axle, for the same reason.

Happy counting!

Machining differential carriers for the Ford IRS

Using a non-IRS limited slip in a 7.5 or 8.8 Ford IRS axle requires the builder to choose between leaving out the axle retainer clips, or machining the differential side gears to accept them.

For those who want to keep their retainer clips, I’ve come by the following diagram for machining the side gears:

Machine work diagram
Machining Differential Side Gears for the Ford 7.5 and 8.8 IRS

I originally found this drawing on TCCOA in a post here, courtesy of 392Bird.  No idea where it originally came from.

The side gear in the drawing is straight cut, unlike the gears used in the Trac-Loc, but that doesn’t really matter.  What’s important here is the bevel machined between the inboard side of the splines and the gear face for the c-clip.  This bevel will allow your IRS retainer clip to compress, so that you can remove the half shaft without having to disassemble the center section to compress the clip.

GearCalc spreadsheet

GearCalc Screenshot
GearCalc by Chuck Sanders

Over the years, I’ve developed my own spreadsheet for doing axle and speedometer gearing calculations.  It started in Excel, and now lives in Google Drive:

GearCalc on Google Drive

Please feel free to copy or download this calculator – like the rest of the site, it is shared under a Creative Commons license.


  • Supports up to six speeds, plus two ranges
  • Calculates speedometer drive gear for Fords, including speedometer error.  (This will work for some other makes as well.)
  • Shows the RPM after shift for each gear shift

How to Get Your Copy:

  • If you’re already a Google Drive user, click the “File” menu, and “Make A Copy.”  This will give you your own copy, which you can edit at will.
  • If you’re not a Drive user, you can click “File” > “Download As” to get a copy in the spreadsheet program of your choice.  You might have to do a little cleanup work to make it pretty again, but everything should work.

Instructions for use:

  • Fill in the (yellow) blanks with your information.  Vehicle and Trans fields are only there so you can keep track of your results if you choose to print this out.
  • Axle field should be obvious.  If you’re using a portal axle (Unimog, H1, aftermarket, etc) you’ll need to put your total axle gearing here, not just your differential gears.
  • Range can be 0 for single range cars, or the ratio in your transfer case for four wheel drives or trucks with splitters.
  • Tire diameter is in inches.  Your equivalent tire revs per mile will show on the right for the tire diameter you enter.  If you are going primarily by a manufacturer’s rev/mi figures, I’d recommend guessing your diameter until you get Rev/Mi correct as the fastest method.
  • Enter your transmission’s gear ratios in rows 11-16.  Unused gears can be left at “0”.
  • If you need revs at a specific road speed, you can easily change the speeds in row 10.  Engine revs below will change to suit the speeds you’ve entered.
  • Entering your shift point in row 38 will show you your road speed for each shift, RPM before and after, and % drop in RPM.

I’ve found this spreadsheet very useful over the years, and hope you do, too!  Please, feel free to leave any questions in the comments below.