CAR TALK\Rustproofing that new car may be doing more harm than good.

Byline: Tom & Ray Magliozzi 

Dear Tom and Ray: My wife and I are in the process of purchasing a brand-new Mercury Villager. Our question revolves around the issue of undercoating and rustproofing. Do we really need to have our car rustproofed? And if we do, who should we get to do it?

- Bob

Tom: We've stopped recommending rustproofing, Bob, for two reasons.

Ray:The lesser of the reasons is that manufacturers have improved their rust protection to some degree. They now dip the car parts in some sort of lactose-free, magnesium-zinc, corn-syrup-based fruit shortening, which supposedly adds some protection against rusting.

Tom: And their "rust through" corrosion guarantees reflect an increased confidence on their part that they've been at least somewhat successful - that's vague enough to be weaseled out of easily.

Ray: The more important reason we don't recommend rustproofing is that it's usually done so badly that it actually makes the car MORE likely to rust, not less likely.

Tom: Let's face it, the guys that do the rustproofing are not recent graduates of the Star Fleet Academy. And they're poking holes in the bottom of your brand-new car, and supposedly squirting chemicals in door panels and body panels. And if they do it right, it can help your car last longer. But if they do a sloppy or incomplete job, one of two things can happen. If they don't put the chemicals where they're supposed to go, you can end up with extra holes, which can let moisture into the door and body panels and facilitate rusting. Or, if they put too much of the stuff in, your drain holes can get plugged up, which also accelerates the rusting process.

Ray: Now I'm sure the Continental Rustproofers United in Defense (CRUD) is going to be all over us for expressing such a blanket opinion. And I'm sure that there are some skilled, honest rustproofers out there. The problem is, even WE don't know how to identify the good ones. If I bought a brand-new car tomorrow, I wouldn't know who I could trust to rustproof it correctly. And I'm in the automotive business - you know, honor among thieves and all that.

Tom: So unless you personally know and trust someone who rustproofs cars, our advice these days would be to keep the car clean and take your chances.

Dear Tom and Ray:Does putting bigger wheels on your car make the speedometer read faster or slower? I've got a bet on this.

- Jim

Ray: The speedometer will read slower, Jim. The speedometer reading is based on how many times the axle turns. With bigger wheels, the axle will turn fewer times to cover the same distance, so the speedometer will think the car is going slower.

Tom: If you don't believe us, put on some larger wheels, bring your car up to the "speed limit" on your speedometer, and drive through your favorite local radar trap (preferably with a smile and a wave). I'm sure the nice police officer will be happy to confirm that you were, indeed, going faster than your speedometer led you to believe. In fact, you'll even get a written confirmation of this should you need it to settle the bet.