Do Truck Toppers Actually Improve Gas Mileage?

So you’ve got your shiny new pickup truck, but after a few days behind the wheel you’re noticing fuel economy figures that are a little bit lackluster. You knew trucks were bad on gas, sure, but this bad? Surely there’s something you can do here. Maybe one of those toppers that people are always putting on their beds to keep belongings secure. Will a bed cap cure your fuel economy woes?

The answer is, unfortunately for you, a resounding it depends. On your truck, your driving conditions, whether Mercury is in retrograde — your very literal mileage may vary. Rigorous instrumented testing is tough to come by, and user experiences vary. One thing, though, is universally certain: When bed caps do make a difference in the fuel economy of a pickup truck, that difference is usually very, very small.

The Fast Lane Truck, an auto outlet with a dedication to getting answers and the budget to actually run tests in-house, tested a bed cap on a Ford F-150. They found that the capped pickup only gained one tenth of a mile per gallon over the same truck with an open bed. Footage of a wind tunnel test on the Ram 1500 shows why the gain is so small: The abrupt rear end of a pickup’s cab sends air flying off, rather than clinging to the bodywork and getting caught up in the bed. 

Looking at a scale model of a pickup in a wind tunnel, both with and without a bed cap, gives more insight as to the lack of hard differentiation. Adding an aero-focused bed cap, with a fastback shape, does prevent a high-pressure zone of air from building in the truck’s bed — only to create a low-pressure zone of vacuum behind the tailgate. A flat-backed bed cap will have the same effect along the full height of a vehicle, creating the same zone of low pressure that causes drag in everything from hatchbacks to vans to tractor trailers. 

When it comes to increasing your truck’s fuel economy, a bed cap won’t make the difference you’re hoping for — and it’ll take more fuel stops than you can count before you actually pay back the investment in the topper itself. If you want to keep your cargo secure, sure, get yourself a bed cap. If you want to improve your truck’s MPGs, though, just go light on the pedals.