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November 02, 2026 4 min read
Don't know what gears are in your rear end? Here's a simple way to find out without removing the differential cover.
Whether you've purchased a classic car, are working on a restoration project, or just want to know what's inside your rear axle, knowing your gear ratio can tell you a lot about how your vehicle will perform.
Your rear-end gear ratio affects acceleration, engine RPM at highway speeds, and how your vehicle puts power to the pavement.
Fortunately, there's a simple way to estimate your gear ratio without taking anything apart.
I'm going to show you how I did it on my Chevy C10 project.
Follow along as I demonstrate the procedure on my C10, using a couple of reference marks and counting revolutions.
You don't need any specialized measuring equipment for this procedure.
A piece of masking tape or a paint marker
A floor jack and properly rated jack stands
A way to safely rotate the pinion yoke
A little patience while counting revolutions
This method works whether you have an open differential or a Positraction rear end, provided both rear wheels rotate together at the same speed.
If you've got a couple of friends helping, have one person turn each rear wheel in the same direction, keeping them synchronized, while you count the revolutions of the driveshaft or pinion yoke.
Once both wheels have completed one full revolution, the number of driveshaft revolutions gives you your approximate rear-end gear ratio.
With the vehicle safely supported, the engine off, and the transmission in neutral, locate a lug nut or wheel stud that you can easily see.
On my C10, I positioned a lug nut at the six-o'clock position and placed a small piece of tape on it as a reference point.
This gives me an easy way to identify when the wheel has completed one full revolution.
You don't have to use tape, but it makes the process pretty much foolproof.
[IMAGE: Lug nut marked at six o'clock]
Next, move to the pinion yoke at the front of the rear differential.
I placed a reference mark at the twelve-o'clock position on the pinion yoke and another corresponding mark on the differential housing.
Now I've got two reference points: one on the wheel and another on the pinion.
These marks make it easy to count complete revolutions without losing track.
[IMAGE: Pinion yoke and differential housing reference marks]
Now comes the easy part.
Slowly rotate the pinion yoke while watching your marked lug nut. Make sure both rear wheels rotate together at the same speed.
Every time the pinion reference mark returns to its starting position, you've completed another revolution.
On my C10, I counted:
One complete revolution
Two complete revolutions
Three complete revolutions
At three revolutions, my marked lug nut hadn't quite returned to its original six-o'clock position.
So I continued turning the pinion until the lug nut completed one full revolution.
It took another half-turn.
My final count was approximately 3½ revolutions of the pinion for one complete revolution of the wheel.
[IMAGE: Lug nut returning to six-o'clock position]
Your rear-end gear ratio represents how many times the pinion rotates for one revolution of the ring gear and axle assembly.
With both axle shafts rotating together, the number of pinion revolutions gives you an approximate gear ratio.
Here's a simple reference chart:
| Pinion Revolutions | Approximate Gear Ratio |
|---|---|
| 3 turns | 3.00:1 |
| 3¼ turns | 3.25:1 |
| 3½ turns | 3.50:1 |
| 3¾ turns | 3.75:1 |
| 4 turns | 4.00:1 |
Keep in mind that these are approximate measurements. Actual factory gear ratios commonly include 3.08, 3.23, 3.42, 3.55, 3.73, and 4.10.
For example, counting approximately 3¾ revolutions could indicate a 3.73:1 gear set.
On my C10, I counted approximately 3½ turns, suggesting a gear ratio in the neighborhood of 3.50:1.
The point isn't to measure the ratio to the third decimal place. It's to get a good idea of what gearing is inside the rear end without removing the differential cover.
Before attempting this procedure, make sure the vehicle is securely supported on properly rated jack stands, the wheels are safely off the ground, and the vehicle cannot roll or shift unexpectedly.
Never work underneath a vehicle supported only by a hydraulic jack.
Both rear wheels must rotate together at the same speed for the counting method described here to give the approximate axle ratio.
Also, take your time when counting. Losing track of a quarter-turn or half-turn can make a difference when you're trying to identify a particular gear ratio.
You don't always have to take something apart to figure out what's going on inside it.
Sometimes a couple of reference marks and a little mechanical knowledge are all you need.
Whether you're restoring a classic car, building a street machine, or trying to figure out what's in that rear axle you picked up at a swap meet, this simple technique can save you some time.
Got a question about your project?
Visit theAsk Troy section atVTwinsToV8s.com and send me your question.
Troy Kane
V-Twins to V-8s
ASE Master Technician | ASE Collision Specialist | Over 40 Years of Automotive Experience
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This site documents real builds, tools, and shop work from my own projects. Some pages are showcases. Some are how-tos and tool reviews. If you’re working on a project and want experienced guidance, I offer one-on-one coaching.