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Gear Ratio and FAQ's

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.

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.

The most important parts of your vehicle are the parts you never see. Most newer vehicles have a high-geared overdrive and are shipped from the factory ‘under-geared’ so that the vehicle runs at lower RPMs on the freeway.

That way, the manufacturer can increase their overall mile-per-gallon average. Even slightly larger tires change the RPM at which your vehicle was designed to shift gears, creating more heat, wearing out your transmission and drive train pre-maturely. This, and adding heavy accessories, also puts more strain on your engine and transmission, greatly reducing your power and fuel economy. By changing the gearing, and effectively the gear ratio, you can easily take back all that power, fuel economy and reduce wear, adding years to your vehicle.

Picking The Correct Ratio

My best advice here is to not trust the charts or a quick guess from a salesman. Let someone who has done and driven many help you. There are many factors including Engine, Transmission, Stock Gear Ratio, Weight, and Driving Style. The worst thing you can do is Re-Gear both of your axles and go drive it and realize it did not change much.

Eighty-Ninety Gear & Off-Road proudly serves the Differential Repair needs of customers in Marietta GA, Kennesaw GA, Atlanta GA, Roswell GA, Acworth GA, Canton GA, and surrounding areas.

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