This is a simple method to determine your rear-end type and gear ratio without opening up your differential. If you’d rather be more precise and open up the diff, we’ll cover that below.
You can easily find your gear ratio with this method and all you need is a jack, a pair of jack stands, and the ability to count.
Before you get too deep into this, be sure to check your rear-end for an info tag. I could be that easy…
If there’s no tag, keep on reading.
Do you have a positraction, limited-slip, or open rear-end?
With the rear of the vehicle safely on jack stands (under the axle is best), simply turn one of the wheels. If the other wheel turns the same way or doesn’t turn at all, you should have a positraction or limited-slip rear-end.
If the other wheel turns opposite of the way the first wheel spins, then it’s probably an open rear-end.
If you have a positraction or limited-slip rear-end:
If you have a vehicle with a positraction or limited-slip rear-end, jack up the rear of the vehicle and get both rear tires off the ground. Now put your jack stands under the axle.
Make a mark on the pinion yoke and onto the rear end with chalk or a grease pencil. Do the same to the tire on the car, so you can see when the tire has rotated exactly one turn.
Now count the number of turns of the driveshaft that it takes (use your chalk marks) to make the rear tire turn one complete rotation (remember, the transmission needs to be in neutral or out of gear).
If your car has an open rear-end:
If your car has an open rear-end, jack up only one rear tire and leave the other on the ground. Now put a jack stand under the lifted side.
Make a mark on the pinion yoke and onto the rear end with chalk or a grease pencil. Do the same to the tire on the car, so you can see when the tire has rotated exactly two turns.
Turn the tire and count the number of turns of the driveshaft after tire two full tire rotations.
Review:
- Posi or Limited-Slip Rear End: Jack both wheels off the ground. Turn one wheel 1 complete turn. Count the turns on the drive shaft.
- Open Rear-End: Jack one wheel off the ground. Turn the wheel 2 complete turns. Count the turns on the drive shaft.
What Is My Rear-End Gear Ratio?
If you counted about 2 and three-quarters turns, it’s around a 2.73 gear ratio.
If you counted just a little more than 3 turns, you have 3.08s.
If you counted about 3 and a quarter turns of the driveshaft, you have 3.23 or 3.25 gears.
If you counted about 3 and three-quarter turns, it’s 3.70 or 3.73 gears.
If you counted just a little more than 4 turns, you have 4.10s.
Editor’s Note: This method won’t work very well on a posi that’s worn-out and won’t spin both tires equally.
This is a quick and easy way to tell what gear ratio is in your rear-end. You might not be able to tell exactly what gear you have, but you’ll be very close.
This method can be handy when looking through junkyards or swap meets. It’s good to be able to figure out what gear ratio you have without tearing anything apart.
Want An Accurate Gear Ratio? Count the Teeth On The Gears
If you want to be accurate, you’ll need to count the teeth and do some math.
Pull the cover from the differential.
Before you start counting teeth on the gear, look for the gear ratio stamped into the ring gear…
If there is no stamp to be found, you need to count the teeth of the ring gear and pinion gear.
With the teeth counted, do the math:
Divide the ring teeth number by the pinion teeth number.
For Example: 41 teeth on the ring and 10 teeth on the pinion = 4.10 gear ratio.
