How will towing affect my gas mileage?

The effect of towing on your gas mileage may make you a frequent visitor to your local gas station.
David McNew/Getty Images

Towing presents a number of challen­ges. Aside from the entirely new set of driving skills you have to master, there are the logistics like tongue weight, gross trailer weight and hitch strength to consider. After you've tackled all of those issues, the last thing you want to worry about is how much the whole setup will cost you at the pumps.

Unfortunately, decreased gas mileage when towing is an unavoidable reality; you have Isaac Newton and his second law of motion to thank for that. Simply stated, force equals mass times acceleration [source: Henderson]. Basically what Newton was trying to say -- in towing terms -- is the heavier the load, the more force you need to tow it. That force, of course, is provided by your engine, so the more force it must exert, the more gas it's going to consume.

Whatever mileage your car is rated to get, you can be certain you'll see that mileage drop in direct proportion to how much weight you're pulling. This is because when car manufacturers devise the weight rating, they're assuming the car will only carry 300 pounds (136 kilograms) of cargo, including passengers [source: fueleconomy.gov]. That doesn't even cover the weight of two full grown men! When you consider that every 100 pounds (45 kilograms) of extra weight in your vehicle decreases fuel efficiency by 2 percent, it's easy to see how towing can be quite the fuel hog [source: fueleconomy.gov].

Compounding this "weight problem" is a little thing called drag, the effect of wind pulling on the vehicle and whatever it's towing, thus forcing the engine to work even harder. Bulky loads like trailers and roof racks present a greater surface area for that pesky wind to pull on. According to a Consumer Reports test, a simple roof rack decreases fuel economy by 5 percent. At highway speeds, more than 50 percent of engine power must be used to conquer drag [source: Consumer Reports].

All of these statistics paint a pretty dismal picture for people who regularly haul their gear around. If a measly bike rack can have that much effect on fuel economy, how are you and your new houseboat going to make it to the lake without having to refuel every few miles?

Before you go selling your pickup in a fit of despair, though, take a look at Is there a green way to tow? for ways to get the best gas mileage when towing. And for a better look at the impact of towing and gas mileage, turn to the next page for some real-life scenarios.

The Heavy Burden of Gas Mileage When Towing

Drag and weight are two main factors that determine your gas mileage when towing.
Drag and weight are two main factors that determine your gas mileage when towing.
Mark Horn/Photonica/Getty Images

As you know from Is there a green way to tow?, there are a number of things that impact your gas mileage when towing. In addition to the weight of your load and how bulky it is, factors like how fast you drive and whether you're using a gasoline or diesel engine will play a role.

While diesel fuel is usually more expensive than gasoline, diesel engines can get 12 to 15 percent more power out of a gallon of fuel, so depending on how much you tow, the trade-off could be worth it. In addition, diesels tend to have more pulling power than gasoline engines, enabling them to tow heavier loads without working as hard [source: Murphy].

And while conventional wisdom says the bigger the engine, the worse the fuel economy, when it comes to towing, bigger may actually be better. Larger, more powerful engines don't see as much of a drop in gas mileage when towing as smaller ones do. They simply don't have to work as hard because they're designed for that extra load. So even though large vehicles never get excellent mileage, when compared to small or medium-sized vehicles that may only get 60 percent of their rated fuel economy when towing, these larger engines may actually come out on top [source: Murphy].

It also matters where you'll be driving. If you encounter a lot of stop-and-go traffic, the drain on your mileage will be greater than if you're traveling on long stretches of highway. Again, this is Newton's doing: According to the law of inertia, objects in motion tend to stay in motion and objects at rest want to stay at rest. In towing terms, a lot of stopping and starting puts more of a drain on the engine's energy use because you have to repeatedly get that heavy mass rolling. But once it's going, such as on the highway, momentum helps to keep it going so the engine doesn't have to keep exerting as much energy as it did initially.

As you can see, because of all the variables involved, it's hard to say with certainty how much of a drop you'll actually see in your mileage when you tow. Experienced towers cite a decrease from 5 to 10 mpg on average and note that they definitely see more of a difference when traveling at higher speeds [source: Tundra Solutions].

At today's gas prices, even 5 mpg is a lot of moolah, but as long as you're not high-tailing it down the interstate at speeds greater than 80 mph (129 kph) with a double-wide trailer in tow, you should be able to manage at least a trip or two. For more on towing and gas mileage, haul yourself to the next page.

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Sources

  • Cook, Miles. "How to Tow a Trailer." Edmunds.com. (Oct. 10, 2008) http://www.edmunds.com/ownership/howto/articles/44921/article.html
  • "Driving More Efficiently." Fueleconomy.gov. (Oct. 10, 2008) http://www.fueleconomy.gov/feg/driveHabits.shtml
  • Evarts, Eric. "Comparing mileage: Not all mpg's are created equal." Consumer Reports. July 25, 2008. (Oct. 10, 2008) http://blogs.consumerreports.org/cars/2008/07/save-gas.html?resultPageIndex=1&resultIndex= 4&searchTerm=towing%20gas%20mileage
  • Henderson, Tom. "Lesson 3: Newton's Second Law of Motion." The Physics Classroom Tutorial. (Oct. 10, 2008) http://www.glenbrook.k12.il.us/gbssci/phys/Class/newtlaws/u2l3a.html
  • "Many Factors Affect MPG." Fueleconomy.gov. (Oct. 10, 2008) http://www.fueleconomy.gov/feg/factors.shtml
  • Murphy, Greg. "Tow Vehicle Considerations." Advancedairhitch.com. FloridaHorse.com. November. 2004. (Oct. 10, 2008) http://www.floridahorse.com/towvehicle.html
  • "Tips for saving fuel." Consumer Reports. April 2007. (Oct. 10, 2008) http://www.consumerreports.org/cro/cars/tires-auto-parts/car-maintenance/tips- for-saving-fuel-1205/overview/index.htm
  • "Tacoma towing MPG." Tundra Solutions Forum: TundraSolutions.com. (Oct. 10, 2008) http://www.tundrasolutions.com/forums/towing/46089-tacoma-towing-mpg/