The Novak Guide to
Gearing & Gearing Math for Jeeps
The reduction in RPM's = reduced powertrain wear and fuel consumption at cruising speeds
Miles Per Hour
Figuring Differential Ratio
Because axle gearing is so crucial, here is the quickest way to figure your Jeep's ratio: Jack up both wheels. Mark the inside of the tire with chalk for a reference point. Mark the differential housing and the pinion's u-joint dust shield with an additional mark. Turn the driveshaft by hand and count the number of turns and partial turns the driveshaft makes to turn the wheel one revolution. The result is the ratio.
Common ratios in Jeeps include: 3.07, 3.31, 3.55, 3.73, 4.10, 4.56, 4.88, 5.38, 6.17.
|It should be mentioned here that lower axle gears (higher numerically) are inherently weaker. The smaller pinion size creates less tooth contact and greater gear tooth strain. It is recommended that moderate gears be used in the axles and low gearing be obtained in the rest of the gear train (i.e., the transmission).|
Equation Summary (for you math wonks)
- Crawl Ratio = Transmission Low Gear x Transfer Case Low x Differential Ratio i.e., 6.68 x 2.46 x 4.88 = 80.2
- RPM = (mph x transmission gear ratio x axle gear ratio x 336) / tire diameter i.e., (65 x 1 x 4.11 x 336) / 33 = 2720 rpm
- MPH = (RPM x Tire Diameter) / (Gear Ratio x 336) i.e., (2500 x 31) / (3.73 x 336) = 61.8 mph