•Q. There once was a black plastic thingamajig hanging under the front end of our Mercury Grand Marquis, right behind the front bumper. We could almost always hear it scrape against concrete parking blocks in parking lots. Recently, it was partially torn away by a parking block. Einstein, aka my husband, who knows nothing about cars except how to put gas in the tank and air in the tires, tore away the rest of it. He claims that it is not needed and that we will get better gas mileage without it. I say that the automotive engineers probably put it there for a reason and that it should be replaced. What say you? Do you know what that thingamajig is and why it was there, and is it OK to leave it off? Oh, and does your brother agree with your response?•

RAY: I’m afraid old Einstein is half right. Maybe he would have come up with “e = mc + 3” or something. He’s close.

TOM: What broke off is an air deflector. It’s a cheap, plastic device that directs air underneath the car in order to improve mileage.

RAY: So he’s wrong about getting better mileage without it (although maybe you’ll get better mileage than with it hanging halfway off!). But he’s right that you don’t really need the thing.

TOM: A primary principle of aerodynamics is “the smoother, the better.” You want to reduce air turbulence. In terms of shapes that do that, think of an airplane, or a bullet. Because when air passes over something smoothly, it creates less drag than if it’s interrupted by baffles and edges.

RAY: Since there are some hard edges and odd-shaped parts under the bumper and at the front of the lower engine compartment, the manufacturer covered them up with a cheap piece of plastic, to send the air under the car instead of right into that stuff.

TOM: Does it make a big difference in your mileage? No. It’s only a factor at higher speeds. But manufacturers work in fractions of a mile per gallon, knowing that they all add up. So to them, it was worth the small cost. Or maybe it was worth the cost for all the money they’ll make later on replacement air deflectors!

RAY: But is it worth it to you? Probably not. Because this wind deflector is ... what’s the nice way to put this? Cheap junk. And because it’s in the direct line of fire of concrete parking blocks, it’s very common for it to break or fall off. Lots of our customers just ask us to tear it all off rather than have to listen to it scraping along the ground.

TOM: Driving without it won’t do any harm. Unless it results in Einstein’s confidence swelling to the point where he attempts to repair something with electricity running through it, and hurts himself. Good luck.

•Q. I have a 2000 Subaru Outback wagon. Recently, the passenger-side daytime running light and low-beam headlight stopped working. When I got out of my car to check that the headlight was out, I happened to bump the front of the headlight casing and — lo and behold! — the headlight came back on. I opened the hood and tried to see if any wires were loose, but it did not appear so, and tugging on the wires does not cause the light to go on or off. So now, whenever I start the car and the headlight isn’t working, I simply get out and give the front of the headlight casing a bump — Fonzie-style — and that usually does the trick (sometimes it takes two or three whacks for it to stay on). What could be causing this, and how can I fix it?•

TOM: You can fix it with a new headlight bulb.

RAY: I’m guessing the filament is cracked and is separating slightly when the bulb cools off. And somehow, when you pound on the headlight casing with your fist, you’re getting the two parts of the filament to make contact again.

TOM: But eventually — probably by the time you read this — the bulb will fail permanently, and the Fonzie trick will only result in a large black-and-blue mark on your hand. Hint: The time to stop banging on it and give up is when you’re putting dents in the hood.

RAY: Here’s the good news: A bulb for this car costs less than 10 bucks. And you easily can change it yourself.

TOM: So, pound away as long as you want. But sometime before you actually break your hand, I’d recommend just replacing the bulb instead.

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