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Front Page - Friday, June 25, 2021

How to pick the right axle ratio for your truck

There are a number of design choices to consider when you’re shopping for a full-size pickup truck. Some are common elements such as bed length, cab size, engine and trim level. But there’s a less-known purchase decision that’s also important if you plan on towing with your truck: Axle ratio.

Understanding what an axle ratio is and which axle ratio is right for you will improve the driving experience in your next truck and give you enough capability to tow whatever trailer you’ve got in mind. Edmunds’ experts put together a guide to answer the most pertinent questions.

What is axle ratio?

The term refers to the gears in the truck’s differential, which is the fixed mechanical gearing that links the rear axle to the driveshaft and then the engine. The numbers relate to the number of times the driveshaft has to rotate to make the driven wheels make a full rotation.

It’s written with the driveshaft rotation first and then the wheel rotation, the latter of which is always 1. You could see, for example, a truck’s axle ratio expressed as 3.21:1 or 3.21.

A full-size truck will typically have a few different available ratios, although the specific numbers can vary depending on the truck and manufacturer. A particular truck configuration will have a standard axle ratio that it comes with, plus a few extra ratios that are available as options.

For example, consider a 2021 Ram 1500 crew cab in the Laramie trim level. It comes standard with a 3.21 ratio and offers optional 3.55 and 3.92 ratios.

Effect on towing

Let’s say you’re shopping for that Ram 1500 crew cab with rear-wheel drive and the 5-foot-7-inch bed length. In that layout, in the base Tradesman trim with the optional V6 turbodiesel engine, the Ram has a standard 3.21 axle ratio and is capable of towing up to 8,230 pounds. But opt for the higher 3.92 axle ratio and that same truck has a towing capacity of 9,930 pounds.

The extra towing capacity comes about because the increased engine spinning helps improve off-the-line acceleration and pulling power when carrying heavy loads.

That difference can be even bigger with a more powerful engine. Take that same Ram 1500 but give it the optional 5.7-liter V8 engine. The standard 3.21 axle ratio then gets you a maximum tow rating of 8,440 pounds. Go with the 3.92 gearing, and the tow rating jumps up to 11,540 pounds – a massive 3,100-pound difference.

Fuel economy trade-off

The axle ratio you go with also will affect your truck’s fuel economy. A higher axle ratio (like the 3.92 we mentioned earlier) will allow for higher towing capability, but it requires higher engine rpm for any given speed and then results in lower fuel economy.

So, to return to the same example, a Ram 1500 with the standard 3.21 axle ratio will get better fuel economy than a truck fitted with the 3.92.

If towing isn’t your primary concern, the standard axle ratio is probably your best choice to balance towing capability and fuel economy. Manufacturers use the standard axle ratio when setting fuel economy estimates, but fuel economy can drop by 1-2 miles per gallon when changing axle ratios.

Getting higher ratios

Typically, on light-duty and heavy-duty full-size trucks, an upgraded axle ratio will be available as a stand-alone option or as a part of an optional towing package. Such packages add upgraded axle ratios along with other equipment to make towing easier.

Features such as four-pin and seven-pin hitch connectors, upgraded cooling via larger radiators and even heavy-duty springs can all help with the task of towing a larger trailer. These packages can be pricy, so be sure to compare the weight of your trailer and the capacity of the truck you’re interested in before upgrading your equipment.

Edmunds says

With so many available engines and axle ratios for pickup trucks today, we recommend downloading vehicle towing guides (also known as towing charts) from manufacturer websites. These will help you determine which engine and axle ratio is right for your needs by matching engines and axle ratios to towing capacities.

Also be sure to check the towing guides for each truck you’re considering. Differences between manufacturers in tire size, engine capability and the number of transmission speeds can all change towing capability.

Just because two trucks have the same axle ratio does not mean they will have the same capabilities.

Travis Langness is a reviews editor at Edmunds