Question: In spite of all the advice I've heard from you concerning diesels, I bought a 2001 VW Jetta TDI. It's a wonderful car, and it's much easier to drive than my girlfriend's gas-powered Passat. Because I'm a "get us off this imported oil" nut and an air-pollution wacko, I am experimenting with running my car on biodiesel as well. So far, so good — 40 to 50 miles per gallon, and plenty of power. However, I have a question about tires. The car was delivered with what most people consider desirable wheels and tires: big, wide R205/55R16s. I want to improve the mileage even further. Can I use tires that will get me better mileage? — Tom
P.S. You are free to slander me, I'm used to it.
Ray: Well, before we slander you, we want to clear up our position on diesels. For the record, we don't hate diesels. We just think they're stinkier, pokier and noisier than gasoline-powered cars. Plus, not every gas station offers diesel fuel, which can be a bummer when the tank's running low.
Tom: However, by all reports we've read (we haven't driven one), the Jetta TDI is quieter and quicker than most diesels of the past. Although it still burns that primordial ooze with sulfur and dirt chunks in it that we call diesel fuel.
Ray: As for the tires, the answer is yes, you can get tires that will help you get better mileage. But I wouldn't throw away the tires you've got. Since you're an admitted eco-freak, throwing away your current tires will waste the energy and natural resources it took to make those tires. Plus, you'll be adding to the used-tire-disposal problem.
Tom: Not to mention the money you'll be wasting, which could be used on "Save the Free-Range Granola" bumper stickers.
Ray: So keep those tires for now, and study up on the topic of "rolling resistance." Rolling resistance is drag caused by the friction of the tires on the road when the car is moving.
Tom: The best way to minimize the rolling resistance of your current tires is simply to keep them properly inflated. If your recommended tire pressure is, say, 35 psi, and you let them go down to 28 psi, you increase your rolling resistance by about 12 percent. That would have a -1 percent to -2 percent effect on your mileage.
Ray: Eventually, when these tires do wear out, you can shop for some low-rolling-resistance replacement tires. By altering the rubber compounds and using certain tread patterns, tire makers have been able to reduce rolling resistance quite a bit. That might increase your mileage by another few percentage points — about the same as hitting a few green lights on the way home instead of red ones.
Tom: And with all the money you save on fuel, perhaps you'll be able to replace all of those gaskets and seals that your biodiesel fuel is eating in your engine.
Ray: Seriously, Tom, ask your dealer if it's OK to use biodiesel (basically used cooking oil) in this car. I commend you for wanting to reduce air pollution, but that Burger King runoff might be doing a number on your engine.
Question: My father-in-law owns a 2001 Ford Taurus SE. He has owned several Tauruses and Sables in the past 10 years, and he insists that the cruise control will hold his speed down when descending a hill. I own a 2000 Taurus and told him that he is full of it. Even after at least one speeding ticket, he still insists that I don't know what I'm talking about and that "at least it USED to hold the speed down." Who's right? — Dave
Tom: Dave, it's not good for family relations to humiliate your father-in-law. So we'll trust that you'll apply the answer we're about to give with all due sensitivity, tact and kindness, OK?
Ray: The old goat is nuts, Dave. The cruise-control system operates the throttle only.
Tom: Now, having said that, I can tell you why he thinks it's braking for him. Let's say you're driving down a long grade, on a highway, for instance. Normally, if you start to go too fast, you back off the gas pedal. But you rarely back all the way off, because that would be jarring. Instead, you back off a little bit and "accelerate less."
Ray: But the cruise control can back all the way off the gas pedal if you exceed the set speed by more than a few miles per hour. And when you let off the gas pedal entirely, you do experience the natural braking action of the engine, which results from the friction of the moving parts and the pistons having to compress air in the cylinders. And that's probably what he's feeling.
Tom: So if you have a kind bone in your body, Dave, explain to the old man that what he's feeling is the natural braking action of the engine when it's not accelerating. Tell him it makes perfect sense that he would experience this as braking.
Ray: And try not to end the explanation by jumping up and down on his sofa and yelling "naah nah nah naaah nah!"
The Magliozzi brothers' radio show, "Car Talk," can be heard Saturdays at 10 a.m. and Sundays at noon on KUER FM 90.1, and on KCPW 88.3/105.1 FM Saturdays at 9 a.m. and Sundays at 10 a.m. If you have a question about cars, write to Click and Clack Talk Cars c/o King Features Syndicate, 235 East 45th St., New York, NY 10017. You can e-mail them by visiting the Car Talk section of the Web site www.cars.com.