Differential Gear Ratio, Positractions and Lockers | Frequently Asked Questions

On this page:

• What gear ratio do I need?
• Do I need posi or a locker?
• What gear ratio do I have?
• Do I have a posi or locker?
• What is this noise…?

 

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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

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Do I need a positraction or a locker?

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.

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What gear ratio do I have?

Jack up one tire if you have anopen diff, or both tires if you have a working posi orlocking differential. Rotate the tire one full revolutionfor posis and lockers and 2 full revolutions for opendiffs. Carefully count the number of full revolutions the driveshaft makes. This is your gear ratio. In otherwords, if the drive shaft turns 3 ¾ turns, you probablyhave a 3.73 gear ratio. Turning the tire for twice thenumber of full revolutions and dividing the drive shaftrevolutions by two will give you a more accurate reading.

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Do I have a posi or a locker?

Put the transmission in neutral and jack up both tires. Turn one tire. If the other tire spins the opposite direction you have an open differential, and if it spins the same direction you have a posi or a locker.

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