Drive it forever

Q: With the price of gasoline going higher and higher, I have a question about fuel economy. If you are driving down a hill, do you save fuel by putting your vehicle in neutral and coasting instead of having your vehicle in drive and keeping the engine's revolutions per minute up? I think that you do; my wife seems to disagree.

A: A generation ago you would have been correct about using less fuel by coasting downhill in neutral. But modern, fuel-injected cars shut the fuel delivery completely off when decelerating, using essentially no fuel. Pop them into neutral, and the engine will use a small amount of fuel to maintain idle.

But you're probably best off staying in gear when going downhill regardless of whether your car has fuel injection or an old-fashioned carburetor. It will keep you from building up too much speed. Worse yet, if the engine should stall, you could wind up with no power steering or power brakes.

Q: I have a black car and it has a few rock chips in the hood. They show the primer, which is a whitish gray. Could you tell me why automobile makers do not use colored primers? This would make those chips less noticeable.

A: Manufacturers use just one color because all of their cars, regardless of what color they're being painted, are dipped in the same vat of primer. Somebody with a silver car might not be happy about black primer, either. Some touch-up paint, properly applied, will not only cover that tattletale gray, it will also prevent the metal from rusting.

Q: I have a 1954 Johnson outboard engine, and the gas tank is sealed, with a full load of gas inside. The gas is now a few years old, and I was told not to use it because it has probably gone bad. Is it spoiled, and if so, why? It has been in the ground for a few million years, so why would it spoil in a sealed tank?

A: That gasoline has been refined substantially since it was crude oil, making it more susceptible to oxidation. And remember, when it spent those millions of years under the ground, there was no air down there. Even though that tank was sealed, there's still some air in there.

But hey, try it! The worst that could happen is that the varnish built up inside will plug up your carburetor, which probably needs to be rebuilt anyway, after being out of service for so long.

I predict the thing will be really hard to start, though.

Q: I recently purchased a Kia Sedona and I discovered an interesting tidbit regarding tire sealants. (Yes I am one of those people who will read the entire manual on a new car!) Anyway the TPMS (or tire pressure monitoring system) makes the following statement: "Notice: Do not use any tire sealant if your vehicle is equipped with a TPMS, as the liquid sealant can damage the tire pressure sensors." In an emergency I would rather pay for a new sensor than have my wife stuck by the side of the road, but it's an expense to consider. In the meantime, we will continue to rely on the cellphone/AAA membership whenever possible.

A: Point well taken. Some tire pressure mounting systems use a separate sensor, while others simply interpolate data about tire rotation from the ABS (anti-lock braking system). The first type will not tolerate use of tire sealants.

Q: I am ready to replace my OEM tires on my 1998 Ford F-150 4x2 Super Cab. I only drive about 5,000 miles a year. I do not haul anything heavy. I use the truck mostly for recreational use. I seldom drive the interstate and only at moderate speeds. I have an opportunity to replace the original tires with the following: P245-75-16 Michelin tires. Can I do so safely?

A: Yes you can.

Mike Allen is a senior editor for Popular Mechanics magazine. Questions should be sent via e-mail to driveit@nytimes.com.