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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. -- Ken

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.

Dear Tom and Ray:

I know this is ethically questionable, so you guys are the only ones I can ask. My BMW is still under warranty -- barely -- for the next 400 or so miles. I was wondering if there is a way, assuming that there is some underlying condition waiting to break, to push my car to its breaking point, thereby allowing me to take it to the dealership and have whatever inevitably would break 20 miles after my warranty is up break now. My goal is to break stuff that's going to break anyway, under warranty, and get BMW to foot the bill so that I don't have to cough up the ridiculous amount of money it would take to fix anything on this car. Any tips for pushing my car to the breaking point in the next 400 miles would be immensely appreciated! Thanks! -- Lisa

TOM: Lisa, you little devil, you. I'm not sure if I should be insulted or proud that you thought of us for this question. Because it is entirely unethical.

RAY: But let's consider it on its merits anyway.

TOM: Sure.

RAY: OK, I've considered it, and it's a bad idea, Lisa.

TOM: I agree. I mean, you could push your car to the breaking point just by lending it to a local teenager. But I think YOU'RE going to end up paying the price, rather than BMW.

RAY: The problem is that it's impossible to know exactly where the breaking point is for any given part. And if you abuse your car for the next 400 miles, what happens if the parts DON'T break?

TOM: Then you've made it much more likely that they WILL break soon -- 450 miles from now, with you footing the bill.

RAY: You're right that the repair bills for a BMW are steep. But the truth is, except for some electrical and cooling issues, BMWs are pretty durable. Nothing of any consequence may be getting ready to break yet.

TOM: So I wouldn't abuse your car, Lisa. Here's a better idea: Find an independent mechanic you trust who works on BMWs. If you don't have one, search the Mechanics Files on our Web site, www.cartalk.com.

RAY: Pay the mechanic to go over your car from stem to stern, as if you were buying it as a used car. If he finds any parts that are in questionable condition, then you can go to your BMW dealer and ask them to check out those parts.

TOM: If a part is legitimately failing, the dealer either will replace the part, or you'll have a record of having complained about the part during the warranty period. And then if it fails 1,000 miles after the warranty expires, they'll be obligated to fix it for you. Unless, of course, they read this newspaper.

(c) 2009 by Tom and Ray Magliozzi and Doug Berman Distributed by King Features Syndicate Inc.

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