Diesel Tech Q&A: Axle-Ratio Selection

2019 Ram 3500HD towing a New Horizons fifth-wheel trailer
The 2019 Ram 3500HD dually equipped with a 3.73 axle ratio has a 2,000-pound higher maximum trailer-tow rating than an identically equipped one running 3.42 gears. Typically, the best fuel economy in a Cummins comes 100 to 200 rpm above the peak torque rpm, which is 1,800 rpm for the high-output version equipped with the Aisin six-speed automatic. Photo: FCA

FEBRUARY 2019
Axle-Ratio Selection and Fuel Economy

I’m debating between 3.42 and 3.73 axles on the new Ram 3500 diesel dually. I understand fuel mileage will be better with the 3.42, but how much better under similar conditions — e.g., pulling a large fifth-wheel? There are lots of factors besides axle ratios that won’t change (wind and tire-rolling resistance, for instance), so I’m guessing the difference will be less than the 3.73/3.42 ratio would indicate.
Michael Brown, West Lafayette, Indiana

A lot depends on how “large” your fiver is and the type of terrain the majority of your travels take you to. The 2018 Ram 3500 dual-rear-wheel (DRW) configurations all vary about 2,000 pounds in maximum towing capacity when it comes to axle ratios, with the “lower” (numerically higher) 3.73 ratio having the higher tow rating than the 3.42. (Ram has not released the full towing specifications for the 2019s as of this writing.)

For example, a 2018 6.7-liter Ram Crew Cab DRW longbed 4WD automatic can tow a maximum of 16,660 pounds when equipped with 3.42 axle ratios, while one with 3.73s can tow 18,660 pounds. If your trailer has an actual loaded weight of more than 14,000 pounds, I’d highly recommend getting the 3.73 axle ratio. The truck will get the load moving quicker and easier with the lower gearing, and will be a much better performer pulling the grades. By that I mean it would be less of a strain on the Cummins having the 3.73 gearing working for it than the 3.42s in high-load, high-demand situations.

Solo Driving and Towing

When it comes to fuel economy, running empty around town or when driving where speeds are less than 55 mph, you’d see very little difference in fuel mileage between the two axle ratios because the transmission will never get the engine rpm high enough to be a factor. When towing, whatever gear keeps engine rpm between 1,800 and 2,000 rpm will give the best fuel mileage and power because that is the rpm window where the Cummins 6.7-liter makes peak torque. With a little time, you’ll find the engine’s “sweet spot” with the trailer you are towing.

The difference in towing mpg between the 3.42 and 3.73 gears at highway cruise speed, under the same conditions, should be less than 1 mpg because of the slightly higher rpm running 3.73s. Vehicle speed actually plays a bigger role in fuel economy than engine rpm. That’s because towing or unladen, mpg drops exponentially at speeds above 60 mph, which is when aerodynamic drag (Cd) really begins rearing its ugly head related to pickups.

That effect of Cd on fuel economy is magnified when you have the face of a tall trailer acting as a wall blocking the wind. If you are a numbers person, here’s the formula for aerodynamic drag: Cd = frontal area × drag coefficient (Cd) × air density × speed squared.

Note “speed squared.” At speeds less than 30 mph, overcoming rolling resistance (i.e., frictional drag) is what burns fuel. At highway speeds above 65 mph, aerodynamic drag takes over, accounting from half to more than two-thirds of the fuel that’s burned as the engine tries to overcome Cd, according to vehicle-manufacturer design engineers and wind-tunnel tests.

Bottom Line

If you are really concerned about maximizing fuel economy, focus more on the right foot than on the right axle ratio.

Gear set for GM HD truck in shop
Axle-ratio selection plays a big role when it comes to towing trailers that are approaching the limit of a tow vehicle’s maximum tow rating. Choose 3.73 instead of 3.42 ratio in such situations because the “lower” (numerically higher) ratio will get the load moving easier and be a better performer in the mountains. Photo: Bruce W. Smith

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Bruce W. SmithA respected automotive and RV journalist and longtime Trailer Life contributor, Bruce W. Smith has held numerous editorial titles at automotive and boating magazines, and authored more than 1,000 articles, from tech to trailering. He considers his home state of Oregon a paradise for RVing and outdoor adventure.


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7 COMMENTS

  1. Thanks very much for your excellent reply. Guess I’ll go with the 3.73. I’m looking at fifth-wheel trailers, probably in the 13,000- to 15,000-pound range, and normally wouldn’t travel faster than 55 to 60 mph. Also thinking of putting a fairing on the truck roof to somewhat reduce aerodynamic drag. Any general thoughts on whether this would be worthwhile doing? Haven’t picked a trailer yet so don’t know how aerodynamic the trailer itself will be.

    • Bruce W. Smith replies: Michael, reducing aero drag while towing trailers has had great success in the over-the-road industry. The difference between big rigs and your heavy-duty pickup is, for fairings and wind deflectors to provide a return on your investment, they must be perfectly matched to the truck and that specific trailer.

      The fairing/spoiler/deflector on the tow vehicle also has to be in very close proximity to the front of the trailer to be effective, within 2 to 3 feet, according to research we’e done on the topic. If the deflector is farther away, the “wash” from the deflector can actually increase drag. It’s better to choose a fifth-wheel trailer that is aerodynamically efficient.

      Consult with the trailer manufacturer to see if they have done wind-tunnel and computer fluid-dynamic studies on the trailer you are eyeing. The other thing to consider is weighing the cost of a deflector mounted on your truck’s roof against the actual fuel you might be saving. My thinking is it’s better to keep the money in your wallet than it is to put a deflector on your truck roof.

  2. I own a 2014 Coachmen Brookstone fifth-wheel that is approximately 35 feet long. I pull it with a 2008 unaltered Silverado 2500 HD longbed diesel with 51,000 miles on it. If you are familiar with this fifth-wheel, it has a very aerodynamic shape, especially at the nose cone. I enjoy 12 to 13 mpg when pulling on a flat surface such as the freeway and 21 to 22 mpg not pulling. Don’t know if that’s the norm for this type of setup, but I believe the shape of my trailer helps a great deal!

  3. Most newer model fifth-wheels today have a fairly aerodynamic front cap. The only ones that don’t are the aluminum-sided ones that are almost flat on the front, which I wouldn’t want. Good luck, as for the weight of a fifth-wheel, try to stay within 80% of the truck’s tow rating; you will enjoy your trailering much more. Towing is not the problem, controlling it in an emergency situation is the elephant in the room. I know we say we are safe drivers and won’t get into an emergency, but with so many people on their cell phone, you never know when someone is going to pull out in front of us.

  4. Just have to make my comment on these wind deflectors that go on the truck. They do not decrease gas consumption as you are still moving the size of object through the wind. You still have the same amount of wind drag hitting the truck as you would the fifth-wheel, just that the truck would then take the hit instead of the trailer. In the event you have a flat front trailer, yes, it would deflect some wind off the front. Don’t look at them to reduce gas mileage, but instead to take some head pressure off the front of your trailer or fifth-wheel. Yes, they have to be installed as close to the trailer as possible as well. Personally, I don’t think they are worth it. I am open to hearing other thoughts from those that have them or did have them. Experience is the best teacher, but unless the trailer is flat-fronted, I don’t see the logic in them.

  5. You do not mention the Ram 3500 dually with a 4.10 rear end, which offers the greatest tow capacity in the lineup. Steady 16 mpg in mountain country, running empty.

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