Differential Gear Ratio, Positractions and Lockers
The following are a few of the most common Differentials Questions we receive. Our staff of Differential Experts is ready to help with your specific axles and differentials questions Monday thru Friday from 8am to 5pm Pacific Standard Time.
What gear ratio do I need?
Differential Gear Ratio determines the number of times the drive shaft (or pinion) will rotate for each turn of the wheels (or ring gear). So if you have a 3.73:1 gear ratio the drive shaft turns 3.73 times for every turn of the wheel.
Gear ratio is calculated by dividing the number of teeth on the ring gear by the number of teeth on the pinion gear. The higher the number, the lower the ratio: a 5.29 gear has a lower ratio than a 4.10 gear. With a lower gear ratio the drive shaft (and thus the engine) turns more for each revolution of the wheel, delivering more power and torque to the wheel for any given speed. Lower ratios are generally desirable when going off-road. Higher ratios are better for freeway driving since they run at lower RPM’s and offer better fuel economy.
Changing tire size affects the final drive ratio. Switching from a 30″ tire to a 35″ tire changes the final drive ratio by about 17%. This may drop the engine out of its’ “power band” and result in poor performance and fuel economy. To restore performance you must change the gear ratio to compensate for the change in tire size. If you originally had 3.07 gears you need a ratio that is approximately 17% lower, such as 3.55. If you want to increase off road performance you might want a 4.10 or lower ratio.
Recommended Engine RPM @ Highway Speed
- 4 cylinder: 2200 – 3200
- V6 cylinder: 2000 – 3200
- Small block: 1800 – 2800
- Big block: 1800 – 2600
- Diesel: 1600-2800
Check out our handy calculators!
For complete listings of the Ring and Pinion Gear Ratios available for your particular vehicle or application, consult our parts catalog located at the top right of this page.
Do I need a positraction or a locker?
Most vehicles come from the factory with an “open” differential. The open differential is designed to propel the vehicle while also allowing for one tire to be turning faster than the other. (During cornering, the tire on the outside of the corner travels a longer path than the inside tire) This design provides smooth cornering with no adverse tire wear. In a low traction situation (i.e.: one tire on mud or ice) the open differential will apply power to the tire with least traction, resulting in tire spin and no forward propulsion.
A limited slip or positraction differential typically uses some form of clutches that bind up the differential, providing traction to the both tires. The clutches will slip to some extent to allow tires to turn at different speeds on corners. Some limited slip differentials are more aggressive than others, and some can be set up or “pre-loaded” more or less aggressively. Limited slip units require a special gear oil additive and may chatter when turning. Clutch packs may also wear with time and require replacement.
Locking differentials come in various forms, all of which provide 100% traction to both wheels. Automatic locking differentials, such as the Detroit Locker or Lockright require no driver input whatsoever. Selectable lockers such as the ARB Air Locker, Eaton ELocker and Auburn ECTED typically perform as an open differential until the driver selects “locked” mode.
A spool has no moving parts, and basically turns the driver and passenger side axles into a single axle shaft. No provision is made for cornering so tire chirp is unavoidable. Spools are best suited for racing-only applications
For complete listings of the Positraction Units and Locking Differentials available for your particular vehicle or application, consult our parts catalog located at the top right of this page.
What gear ratio do I have?
Jack up one tire if you have an open diff, or both tires if you have a working posi or locking differential. Rotate the tire one full revolution for posi’s and lockers and 2 full revolutions for open diffs. Carefully count the number of full revolutions the driveshaft makes. This is your gear ratio. In other words, if the drive shaft turns 3 ¾ turns, you probably have a 3.73 gear ratio. Turning the tire for twice the number of full revolutions and dividing the drive shaft revolutions by two will give you a more accurate reading.
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