Rear Differential 101: What it All Means

By Trevor Dorchies | August 20, 2012
Over the past decade or so, the big three truck makers (Ford, Chevrolet, and Chrysler) have seen towing capacity skyrocket to never-before-seen levels. For example, since debuting in 2003, the GMC Sierra HD has seen its towing capacity grow 43 percent from 16,100 pounds to 23,100 pounds. So how do you get all the weight moving from a standstill? First, it’s important to understand how a rear differential works and know where it’s located. Look at the rear axle of any pickup on the road; that big bubble at the center of a truck’s rear axle is where the rear differential is located. The rear differential is tasked with distributing power produced by the vehicle’s engine to the rear wheels. The basic function of a differential is to allow the inside and outside wheels to turn at different speeds, essential to making a car drivable around turns, since the outside wheel needs to travel farther, and thus spin faster. A differential is classified by the ratio of the ring gear, which turns the wheels, to the pinion gear from the driveshaft.  For example, if a truck’s rear axle has a ratio of 3.08:1, this means the ring gear has three times as many teeth than the pinion gear from the driveshaft. Generally speaking, the bigger the number, the quicker the engine's torque can get to the ground, and thus increase the amount that a truck can haul. The downside is fuel economy; those bigger ratios mean lower fuel economy.
Today, truck makers offer axles with different gear ratios to appeal to select owner’s wants and needs. If fuel economy is important to you, an axle with a lower gear ratio will make for better fuel economy.  If you do a lot of towing, having a higher gear ratio will make way for a higher towing capacity and quicker starts when at a complete stop.  Normally, light-duty trucks have an axle ratio in the ballpark of 3.0:1 and 3.7:1, while heavy duty trucks have a higher ratio that’s closer to 4.10:1. Arriving at a rear differential calibration is the result of months of testing like accelerating from a standstill, transmission tendencies and fuel economy with towing and thermal management being looked at heavily. So how much of a difference does a rear differential actually make? When outfitted with the proper drivetrain configuration, a truck can tow around 9,000 pounds with a 3.73:1 ratio or 14,000 pounds with a 4.10:1 ratio. Picking the proper axle for you depends on how much combined weight (of the truck and trailer) you’ll be pulling on a daily basis. If you use your truck as a daily driver and occasionally haul mulch or sand home, a lower ratio will do just fine. If you’re towing a backloader to and from the worksite everyday, a higher ratio will be a better fit. Check out the video below that also explains the inner-workings of a rear differential. Source: General Motors, YouTube