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My father wants me to do an experiment with our...

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Dear Tom and Ray:



My father wants me to do an experiment with our pickup truck on our
upcoming drive from Seattle to L.A. He believes leaving the tailgate down
will improve fuel economy by three to five miles per gallon. We have a
3/4-ton Dodge diesel. I remember seeing an article from Texas A&M a few
years ago that claimed the tailgate position made no difference in fuel
economy, and in fact, leaving it down may reduce mileage. By the way, my
father is a retired aerospace engineer, and has included such factors as
"drag coefficient" in his calculations. Who is right? -- Kathy

RAY: We want to be perfectly clear about one thing, Kathy. We have
absolutely no idea who's right. But of course, that won't stop us from
speculating!

TOM: I'll admit that I've never seen any research on this issue, but it
makes sense to me that if you open the tailgate, you eliminate a vertical
surface. And when you eliminate a vertical surface (i.e. a wall) that
stretches all the way across the back of the truck, it makes sense to me
that you reduce the drag and improve your mileage.

RAY: But, since this is pure speculation on my brother's part, Kathy, it's
important to remember two things: 1) that aerodynamics is a very
complicated science, and air doesn't always move in predictable ways, and
2) that my brother frequently has his headlight in his taillight socket.

TOM: But, Kathy, you and your father are the perfect people to do this
research. Just drive down to L.A. with the tailgate up, and drive back to
Seattle with the tailgate down. And then compare the mileage. And your
father, being a scientist, can keep the log book and account for the
temperature, barometric pressure, lead-footedness of each driver, and
anything else than can throw off your gas-mileage statistics. Then mail us
your results, or e-mail them to us through our web site (cartalk.com), and
we'll publish the findings for all of our readers.

RAY: I know that they do sell a plastic "netting" that's designed to
replace tailgates on pickup trucks for this very reason. The netting lets
air through, and supposedly increases mileage. But I'd be very surprised if
these nets boost mileage as much as three to five miles per gallon. If they
did, someone would be making nets to replace windshields!

TOM: And before you begin your research, Kathy, there is one more thing you
should consider. That is the tailgate's role in keeping your stuff from
falling off the back of the truck.

RAY: Although if it turns out that Dad's right, then you can use all that
money you save on gas to replace all the stuff you lose on your way back
from L.A.!




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