A clean car won't get better gas mileage

~Dear Tom and Ray: A friend of mine says I am wrong, and I was hoping you would have the answer. Whenever I go on a long trip, I always wash and wax the car before I start, and as the bugs collect, I hit carwashes as needed along the way. Besides, it is great driving a nice, pretty car. My reasoning is that a clean car has less drag, so there is a small improvement in gas mileage. My friend asked me if this was supposed to be a joke. He says the actual difference in gas mileage would be too small to matter. Can you comment? -- Steven

Ray: If it makes you feel good to drive a nice, clean car, Steven, then you should absolutely do it. But you're not saving a measurable amount of gas that way -- not unless you're cleaning off some enormous, mutant, three-dimensional bird dropping in the shape of airplane flaps.

Tom: While washing the car won't hurt, you'll certainly spend more on the carwashes than you'll ever save on gasoline. I mean, the special "Gold Service," with underbody wash and the air-drying by six exhaling former Soviet ballerinas, has to run you at least $12.95 a pop.

Ray: Your time would be better spent making sure your car has been serviced, your tires are properly inflated, your windows are closed and you drive at the speed limit.

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~Dear Tom and Ray: What is the primary reason why car manufacturers have switched to lighter-weight oils? I've heard several explanations. But once and for all, what is the real reason? -- Dan

Ray: They've switched over to lighter-weight oils because the guys loading in the cases at Kmart were continually complaining of backaches, Dan.

Tom: Actually, the main impetus for using lighter (lower-viscosity) oil is to improve gas mileage.

Ray: The thicker, or more viscous, the oil, the harder it is for the moving parts of the engine to "push" their way through it. The more resistance those parts get, the more fuel that is required to make them move. So, by lightening up the oil, you make it easier for the parts to move, and you reduce your fuel consumption.

Tom: It used to be that thicker oil was used because it provided more protection to those moving parts, especially at higher temperatures, when all oil thins out. But oil technology has improved so much that today's 5W30 oils provide even more protection than yesteryear's thickest glop.

Ray: It's also true that the spaces between parts have gotten smaller over the years as quality has improved. And those big, old, fat, porker oil molecules can't get in there as well as the thinner ones.

Tom: In the old days, you used to be able to fit, say, a Maine coon cat in the spaces between the bearings on an old Chevy. But today the spaces are measured in fractions of a millimeter. And today's thinner oils are exactly what's needed to get into those spaces.