The axle ratio on a truck serves many purposes, from multiplying torque when accelerating from a stoplight to highway cruising with easier-going RPMs. Also called the gear ratio, axle ratio affects a truck’s firepower and fuel economy. So, what is axle ratio? Here’s what you need to know and why it makes or breaks a truck.
What is an axle ratio?
Let’s start with a definition. CJ Off-Road, a reference site for car enthusiasts, defines axle ratio as the “number of revolutions the output shaft or driveshaft needs to make to spin the axle in a complete turn.” A truck’s transmission and differential both have gear ratios for similar reasons. The gear inside the differential, in the rear end of the vehicle, takes the rotational movement of the driveshaft and converts that force to the axles and rear wheels.
The driveshaft spins a cone-shaped gear inside the differential housing. This cone, called a pinion, has thick metal teeth, which, as the pinion rotates, mesh with the teeth on the ring gear, spinning and transferring power to the axles.
The number of teeth on the ring gear divided by the number of teeth on the pinion gear equals the axle ratio. Car sites often list gear ratios as one number—for example, 3.73, 4.10, 4.56—but they’d be more accurately listed as 3.73-to-1, 4.10-to-1 or 4:56-to-1. This means for every complete rotation of the rear wheels, the driveshaft rotates 3.73 times or 4.56 times.
Why does axle ratio matter?
Axle ratio matters, but not to every driver. Axle ratio probably seems superfluous to some vehicle owners. The driver of a manual transmission Toyota Corolla, for example, probably doesn’t need to know that its axle ratio is 3.94:1—there’s no practical use of a Corolla that would warrant changing the factory axle ratio.
However, gear reduction increases torque while lowering speed, improving a truck’s towing and off-roading abilities while reducing fuel consumption. If a truck doesn’t have enough power to get a boat out of the water, a higher axle ratio could help. If a truck’s highway fuel consumption is high, a lower axle ratio would reduce engine RPMs and fuel use.
Think of an old 10-speed bicycle. The bike in first gear is much like a 4.56-to-1 rear axle ratio, providing plenty of torque for acceleration, but cruising would be exhausting. A 2.73-to-1 rear axle ratio is similar to the bike’s 10th gear, providing poor acceleration but effortless cruising with a much higher top speed. Automakers attempt to provide a compromise of attributes when creating a truck’s rear axle ratio, with options to reduce or increase the ratio based on the buyer’s preferences.
Where can I find my axle ratio?
Wondering how to find a vehicle’s axle ratio? There are a few places to look.
The axle ratio should be on the vehicle’s build sheet. Look through all the included options to find which gears were ordered. Accessing the build sheet might be more difficult when buying a used car, but there are options in that case.
Locate the VIN. Enter the VIN into an online decoder to try and break down the vehicle’s major options.
Look for RPO or build codes. Examine the driver’s-side doorjamb or under the center console, as these are common places for the codes to hide. For example, a GMC Sierra might have a G75 code, which shows the 3.62-to-1 rear axle ratio.
Some manufacturers place a helpful tag or sticker on the differential housing. The stickers display the factory axle ratio.
If you can’t find any of the above information, there are more ways to find your axle ratio.
How to calculate axle ratio
There are three main ways to calculate your axle ratio—one option is more precise, while the other two are simpler.
Calculate the rear axle ratio by getting under the truck. Remove the differential cover and drain the gear oil, then remove the pinion and ring gears. Count the teeth on both gears. The number of teeth on the ring gear divided by the number of teeth on the pinion gear determines the ratio.
Get a rough axle ratio estimate. Raise the rear axle onto jack stands so the tires are off the ground. Grab a paint pen or piece of chalk and place a small but visible mark on the tire. Then mark the driveshaft. With the vehicle in neutral, rotate the driveshaft until the tire makes a full rotation. The axle ratio is determined by the number of times the driveshaft spins per one tire rotation. Search the make, model and the estimated gear ratio online, and results should offer a more precise comparison.
Use an online gear ratio calculator. Take note of the engine’s rpm while driving at a steady speed. Enter the rpm and miles per hour, along with the tire diameter, and the calculator will estimate the vehicle’s axle ratio.
Common axle ratios
With a variety of cars and trucks on the road, axle ratios differ as much as the exteriors of a Toyota Tacoma and Tesla’s Cybertruck. Here’s a look at a few of the more common axle ratios available—and why a vehicle owner might want that ratio.
3.15 rear axle ratio: Common in the Ford F-150 equipped with a five-liter V8 engine, this ratio offers fuel economy on par with other V6 engines.
3.21 rear axle ratio: Found in the Dodge RAM 1500, this axle ratio is an economical choice for excellent gas mileage on the highway. The Chevrolet Silverado offers a nearly identical set, with a 3.23-to-1 ratio.
3.73 rear axle ratio: This is the highest gear ratio available on the Ford F-150, offering maximum acceleration and towing power.
3.92 rear axle ratio: Optioned in the Dodge RAM 1500’s Max Tow Package, select this ratio for heavy hauling.
Can (and Should) I Change My Axle Ratio?
Notice a distinct lack of torque when accelerating from a stop, especially when towing? Consider new gears that provide a different axle ratio.
Gear sets are fairly affordable among vehicle modifications—a gear set is roughly one-tenth the price of a supercharger, which is another part that increases torque—usually costing around $500 for a complete kit. However, swapping out gears will take a few hours and requires some mechanical knowledge and skill. A mechanic will likely charge hundreds of dollars for labor.
The rear axle ratio determines if a truck is a capable tow beast or a comfortable highway cruiser—or both. Consider the available axle ratios when buying a vehicle. Consider changing the axle ratios if the current truck’s setup doesn’t match its usage needs. This process can be pricey, but changing the axle ratio can dramatically change the truck’s utility.