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## Tailgate Flap

Science brain teasers require understanding of the physical or biological world and the laws that govern it.

 Puzzle ID: #23752 Fun: (2.6) Difficulty: (2.31) Category: Science Submitted By: drussel3 Corrected By: boodler

While buying his new pickup truck, Homer the penny pincher was asked if he wanted to purchase a tonneau cover for the bed of his truck. The salesman told him it would improve his gas mileage with better aerodynamics. While Homer was interested in saving gas, he couldn't see paying all that money for the cover, so he declined.

When Homer got home, he considered the flat vertical tailgate at the back of the bed. By removing this tailgate from his pickup, he surmised, he would reduce the air resistance of the truck, thus giving him better gas mileage without having to buy one of those expensive covers.

Did he make the right choice?

(Note: A tonneau cover is a soft or hard cover that is secured down over the open bed of a pickup truck.)

The best fuel economy (aerodynamic) solution is to use a tonneau cover. The next choice would be to drive with your tailgate up, and the least desirable choice would be to drop, or remove your tailgate. Read on below for the explanation.

Logic would leave you to believe that by lowering or removing the tailgate, you would improve the air flow over your pickup truck. But with aerodynamics, logic is not always correct.

Removing or lowering of a tailgate actually increases its aerodynamic drag, and increases its lift. Both of which will decrease its fuel economy. What happens is, air in the bed of the truck will slowly swirl around in the bed of the truck, creating a pillow or bubble of air. The approaching air over the top of the cab will then flow easier over this swirling bubble of air. Thus you would have a more aerodynamic flow over the vehicle.

According to an SAE (Society of Automotive Engineers) paper, the average impact on a truck, based on a vehicle lifetime of 100,000 miles will be (as compared to a truck with its tailgate up):
> Decrease of 122 gallons of gas with the tailgate off
> Increase of 243 gallons of gas with a tonneau cover.

At today's gas prices (\$2.00), that is a difference of \$730, comparing a covered bed versus removing the tailgate.

So, keep your tailgates up, or better yet, buy a tonneau cover. Those cargo nets don't work either.

Safe driving.

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