When you're shopping for a pickup truck, you might notice different axle ratios offered as options and wonder which one to choose. You might also wonder if it really makes a difference. For many truck buyers, the standard axle ratio will probably work fine. But if you plan to pull a trailer or haul heavy loads, it's a good idea to understand what the axle ratio is and how it affects your truck's performance and fuel economy.

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Understanding Axle Ratios

Automakers build trucks with a range of optional axle ratios. The term refers to the gears in the truck's differential, which is a mechanical device that links the rear axle to the driveshaft and then the engine. Technically, the number should be expressed as a ratio, such as 3.55:1, meaning the drive shaft turns 3.55 times for each turn of a wheel. But that ratio would most commonly be referred to as "3.55" or simply "three fifty-five."

The key to understanding gear ratios is to remember that, as the numerical ratio goes up, towing capacity increases but fuel economy goes down.

Four-wheel-drive trucks will have a ratio in the front axle's differential that closely matches that of the rear. Unless the truck's window sticker lists an optional axle ratio, it will come with a standard axle ratio that's selected by the manufacturer.

The key to understanding gear ratios is to remember that, as the numerical ratio goes up, towing capacity increases but fuel economy goes down. So a truck with optional 3.73 gears, for example, will tow a heavier trailer than one with 3.55 or 3.21. But it will also use more fuel in all situations because the engine will rev higher.

Standard Axle Ratios

The standard axle ratio for Chevrolet pickup trucks is chosen to balance performance, capability and fuel economy, according to the company. The standard ratio also is biased more toward fuel economy. In contrast, optional ratios tend to provide more towing capability.

If you are constantly towing, the highest ratio possible is what you're after. The choice of a higher axle ratio will only knock off about 1 mile per gallon from the truck's fuel economy. But since pickup truck fuel efficiency is low to begin with, even a 1 mpg reduction is significant.

The "drive cycle," which is how, how fast and where you drive, is also an important consideration when selecting the right axle ratio, according to truckmaker Ram. For lower speeds and rural driving, a higher gearset will add capability and a more spirited driving experience without too much damage to the final mpg number, according to Ram. But if there is a lot of high-speed driving, the buyer might want to consider a numerically lower gearset.

Axle Issues to Consider

One tricky thing about choosing an axle ratio is that the EPA fuel economy information on the window sticker primarily applies to the "base" or standard axle ratio, even though that particular truck might come with an optional axle ratio, says Dan Edmunds, Edmunds' director of vehicle testing. Therefore, any axle ratio you see listed on the "options" side of the window sticker will lower the fuel economy figures printed on the very same sticker.

Another difficulty is that optional axle ratios (and their effects on towing) cannot be compared across truck manufacturers, Dan Edmunds says. That's because each truckmaker may use a different tire size, and the tire itself acts as the final "gear" in the system. It's best to compare the axle options within a single brand to see how each one affects the tow ratings listed in that brand's towing guide, he says.

As you shop for your truck, you might hear axle ratios with lower numbers referred to as being "tall" gears and those with higher numbers as "short" gears. To more easily remember what this means, think of a tall person who travels a greater distance with each step. Similarly, a tall gear moves the truck farther with each turn of the engine. Trucks with tall gears deliver better gas mileage because their engines rev lower at a given road speed. However, taller gears also reduce the torque — or power — so you can't tow heavier trailers or haul heavier loads.

Ram recommends that truck shoppers look at the towing and payload tables on its website. They're there to help customers select the right powertrain for their specific needs. As truck manufacturers produce transmissions using more gears, the axle ratios will also change. For example, a transmission with more gears might allow a truckmaker to offer a taller rear axle ratio (a 3.55 instead of 3.73) and still offer improved towing and hauling capabilities.

Before you leave for the dealership, take a moment to think about how you are going to use the truck. When in doubt, default to a higher numerical gearset. While it will lower your fuel economy slightly, it will also mean you will be more comfortable while hauling and towing over long distances.