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Differential Gears: The Optimum Choice

Low-end grunt vs. top-speed power

Everyone who tinkers with cars knows the phrase "higher numerically, but lower" when referring to ring-and-pinion axle gears. But what does this really mean?

Power Transfer

Unlike electric and steam engines, which have full power throughout their complete rpm ranges, the internal-combustion engine—whether fueled by gasoline, diesel, propane, hydrogen or whatever—is extremely weak off idle. For this reason, internal-combustion engines require rpm multiplication, hence the use of transmissions and differential gears (the focus here). Differentials, regardless if the vehicle is front-wheel drive, rear-wheel drive or four-wheel drive, perform two tasks simultaneously: torque multiplication and transferring the flow of power from the vehicle's longitude to its latitude. Driveshafts twist in the same direction as the engine, while the wheels rotate at a 90-degree angle to the driveshaft to propel the vehicle either forward or backward. The differential changes the driveshaft's "left/right" rotation to the "forward/backward" rotation of the axles through the ring and pinion gears, simultaneously multiplying the rpm of the tires by the gear ratio. This is torque multiplication—essentially giving the physical force generated by the engine more leverage.

Cars with transverse-mounted engines—ones that sit crosswise in the engine compartment—usually utilize a transaxle, so they don't have ring and pinion gears. Their engine power is already turning upon the correct axis, so they only need transmission gears for rpm multiplication.

Gear Math

Differential gear ratios are determined by finding the ratio of teeth in the ring gear compared to the pinion gear (with the pinion gear having the smaller number of teeth). Gear ratios for most popular differentials are almost unlimited, and can run the gamut from around 2.56:1 in lightweight econoboxes to about 8.00:1 in large motorhomes and heavy haulers. But we're not interested in either of the two extremes; we're interested in the gears designed for street or off-road performance: 3.50:1 to 5.38:1.

Lower-geared/higher numerically, refers to a gear ratio that puts more power to the road by causing the engine's rpm to be greater per each revolution of the drive wheels. This is more torque multiplication than with a smaller rear-gear ratio. It allows for more towing capacity (as in towing a boat, travel trailer, or hauling a heavy load) and faster acceleration through application of more torque.

As an example, let's take a gear ratio that was once called the "six-cylinder gear" (back when everyone wanted V-8 power): 4.10:1. To make this simple, we'll use a transmission gear that's straight-through, or 1 to 1, which is usually Fourth gear in today's five-speed overdrive manual transmissions, or Third in a four-speed automatic. For this exercise we'll also ignore the tires' diameter because we're just talking the rpm of the various components, not miles per hour or mileage. With a 4.10:1 ratio and an engine that's twisting at 4,000 rpm, the drive wheels are turning at almost 976 rpm, or 4,000/4.1 = 975.61. This is why the differential gear ratio has a great deal to do with a vehicle's top speed. Lower gears mean lower top speeds; the vehicle is limited by an engine's maximum rpm. Lower gears, however, also mean that a vehicle will get to its (lower) top speed more quickly because the engine can overcome the vehicle's inertia easier with greater torque. Obviously, the tires play a big role here, since larger tires change the final gear ratio. (For example, a dragster's tires grow as their rpm increases, which changes the gear ratios for a higher top speed while still giving the higher torque off the line.) Larger tires can also increase drag or rolling resistance, but that's another story.

Now that you know what "lower/higher numerically" and vice versa mean, you have to decide whether you want better low-end tug (low gears) or higher top-end performance (higher gears). You may want to improve mileage, which means in most cases higher (numerically lower) gearing, or improve your 0-60 sprint with lower gears (numerically higher). Axle gearing is a crucial part of the performance package, and it determines how the engine's power is put to use.