Q: I have a question about fuel economy. If you are driving downhill, do you save gas by putting your transmission into neutral and coasting instead of having your vehicle in drive?

I think that you do, but my wife seems to disagree. Can you give me the correct answer, so that I can tell her that I’m “Mr. Right,” as usual?

A: How about “Mr. That Depends?”

The engine isn’t braking the car going downhill if the transmission is in neutral, so fuel economy would seem to be high.

But if you think the engine is still using fuel while coasting downhill in drive, you’re laboring under a misconception. Most fuel-injected cars turn off the fuel delivery completely as soon as you lift your foot from the accelerator. They still burn fuel when idling in neutral, though, so there’s no savings there. In fact, while the amount of fuel burned at idle during, say, a couple of miles of coasting downhill is small, it’s still more than zero and thus you’ll use more fuel in neutral than you would in drive.

In short, if you’re driving a modern, fuel-injected car, you’re wrong and your wife is right. An older, carbureted car would suck gas through the engine while coasting in gear, though, even if you turned off the ignition switch, so if that’s the kind of car you have, you’re right and your wife is wrong.

Q: How do you build a 9-volt battery connector to the cigarette lighter, to prevent having the car radio, clock and computer reset whenever I replace the battery?

A: Get a cigarette-lighter plug and a 9-volt battery connector. Solder the positive wire from the battery connector — probably the red one, it will be marked with a “plus” sign — onto the center, positive terminal on the lighter plug.

Solder the negative wire — marked with a “minus” sign — onto the outer, negative terminal on the lighter plug.

Q: Do HHO kits or water-for-fuel kits really work? Do they actually improve gas mileage? Has Popular Mechanics done any studies on this?

A: Been there, done that, wore out the T-shirt. Short answers: No, no and yes.

Q: Can you resolve a long-standing disagreement I have with my father about rearview mirrors?

He insists on adjusting them so that he can see only his own rear fenders in the outside mirrors. I like to set them so that I can see just the edge, not the whole fender filling half of the mirror.

A: I had a long chat over dinner with an engineer from a major auto company, a couple of years back, about this very subject. In addition to being an engineer, this guy holds a Ph.D in psychology, specializing in human factors and ergonomics. We agreed that people often don’t adjust their rearview mirrors correctly.

The interior mirror should be set to see straight back along the road, as most people do. But most drivers set the side mirrors to duplicate this view. The engineer suggested that the proper use of these mirrors is to let you visualize the blind spots flanking your car, not to see straight back.

Here’s his procedure for adjusting the mirrors, and it’s what I have always done myself:

Adjust the driver’s seat to the correct position, and then lean your head and torso over to the left until your head touches the window glass. I know, that seems too far, but trust me. Now adjust the left-side mirror outward until you can barely see your own fender in it.

Next, lean over to the right a similar distance, and adjust the right-side mirror so that you can barely see your own rear fender.

Sit up straight. Your mirrors are now set to allow you to see other cars in your blind spot. You won’t need to swivel your neck around to be aware of traffic hiding at 60 mph right beside you.

And the interior mirror will still let you see the traffic directly behind you.