Dear Tom and Ray:
Can you help settle a bet with my dad? He has an SUV, and he frequently goes and gets it buffed and waxed. It runs about $100 each time. I asked him why he does that. He said because it gives him "better gas mileage." I mean, come on! He says "it is a proven fact." I don't believe him. So my question is: Does it improve your mileage, and is it a proven fact? Thank you for your time.
Ray: Ken, I think your old man is confusing "a proven fact" with "something he saw on the Internet."
Tom: Or in our column!
Ray: Right. Real "proven facts" have something called "citations" attached to them. A citation tells you which legitimate, scientific paper claims to have proven the fact.
Tom: So if you ask your dad for the citation on his proven fact, I suspect it'll ultimately be traceable to the American Car Wash and Car Wax Association.
Ray: There's no doubt that aerodynamics play a huge role in mileage. But unless your dad is parking his SUV under a tree that's home to, say, pterodactyls who leave large, three-dimensional droppings on his hood, I don't see how washing the vehicle could make any meaningful difference in his mileage.
Tom: While air generally does flow better over a smooth surface, if you look at aerodynamic photographs or videos, you'll see there's a thin layer of air that's "attached" to the vehicle's surface. It's called a boundary layer, and it doesn't really move when air moves around the car.
Ray: So, most small particles of dirt (except for the aforementioned pterodactyl droppings) would be inside that boundary layer, and wouldn't change the car's aerodynamics (boundary layer citation: Ludwig Prandtl, Aug. 12, 1904, Third International Congress of Mathematicians, Heidelberg, Germany).
Tom: Where'd you find that?
Ray: I saw it on the Internet.
Tom: Well, if it's any consolation, Ken, tell your father he's got the best-looking SUV on the block. And if that makes him happy, he should keep cleaning his car. But if he wants better mileage, he should make his next car something smaller than an SUV.
(Car Talk is a nationally syndicated column by automotive experts (and brothers) Tom and Ray Magliozzi. Write to them at the Car Talk Web site.)