Determine the correct gear ratio by using the formula.
Ratio = (Ring Gear Count/Pinion Tooth Count) x (Bull Gear Tooth Count*/Spur Shaft (Cross Shaft) Tooth Count*)
*Mack Models Only
If it is a rear differential (with the exception of Mack), the tooth counts are stamped and can be found at the threaded end of the pinion gear. For front differentials use the ratio calculator.