The Novak Guide to
Gearing & Gearing Math for Jeeps
The reduction in gearing = an increase in torque at the tires
Note here that automatic transmissions offer the benefit of gearing built into the fluid drive torque converter. One can multiply an automatic transmission's low gear by a factor of 1.5 to 2.
Crawl ratios in the 60's are quite capable; in the 80's, very respectable, in the 110's, impressive and anything beyond the 130's is usually considered the point of diminishing returns.
The reduction in RPM's = reduced powertrain wear and fuel consumption at cruising speeds
A good RPM range for a V6 or V8 is 2200-2700. Fuel economy suffers the higher it goes. Lugging the engine is a risk if it goes too low. This is useful as many Jeep enthusiasts do not want to lose the roadability of their vehicles.
While an individual wanting a better cruising speed may automatically think of an overdrive transmission, keep in mind the effect of larger tires and/or a taller set of axle gears, which are easily compensated for with good, low gears in the transmission and the transfer case. These points considered, it is more important to keep in mind the gearing span of the transmission instead of whether or not it has an overdrive feature, i.e., a T18 has a gearing span of 6.32 vs. an NV4500 at 6.07.
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).
Converting from P Metric Tires to Inch Diameter
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