I was told by a friend you can purchase an inexpensive fuel economy gauge that tells you your gas mileage. Is this true? How could it work? Where can I buy one?

– Jeremy Slocum,

Monte Sereno, Calif.

It sounds like your friend was describing a vacuum gauge, which could be installed on or under the instrument panel. Gasoline-powered engines create vacuum (actually reduced pressure) in the intake manifold due to the pistons attempting to draw in air and the throttle blocking the way. At idle, and under light load (with the throttle barely open), manifold vacuum is fairly high – about 18 inches, as read on the gauge. As greater engine load occurs, when you’re accelerating or climbing a grade, the throttle is opened and vacuum falls. Vacuum at part throttle might be about 10 to 15 inches, at five to 10 inches under heavier load, and it’s at zero at full throttle.

As you can see, the more aggressive the throttle opening, the lower the vacuum. Watching the gauge while driving, with an eye still on the road, encourages one to moderate acceleration to a desired rate, and avoid unnecessary speed-up/slow-down maneuvers.

Actual fuel economy isn’t indicated, but after seeing the results of various driving styles, mileage gains can at least be inferred. Driving as if there were an egg between your foot and the accelerator pedal should produce a fuel economy gain of around 10 to 15 percent. The electronically controlled throttle on many newer vehicles helps in a similar way, by minimizing unnecessary fidgeting of the throttle.

To browse a nice selection of under-dash vacuum gauges, go to www.jcwhitney.com and use the keywords “vacuum gauge.” Installing one will involve poking a hole (carefully) in the firewall, to connect the gauge’s slender rubber hose to the engine. Also, don’t forget to check that your tires are inflated properly, as this improves fuel economy, too.

My left-side taillight has quit working on my GMC pickup. The bulb was checked and it’s OK. When I mentioned that it had a short to my friend, he rolled his eyes. He said a short wouldn’t cause this. If not, what did?

– Sal Molino, Seattle

Your friend is right, and it sounds like he/she has the right stuff to fix it! Short circuits are rare. They are an unwanted connection between the “hot” side of an electrical circuit and the ground. Other, less likely types of shorts are possible as well. In your case this would have likely blown the taillight fuse, taking out both taillights, front parking lights, the license plate light and possibly the turn signals.

A far more likely scenario is excessive or infinite resistance in a nearby connector or the bulb socket. This could be a loose or corroded terminal, or perhaps a loose or broken ground connection. Does the turn/stop lamp work on the left side? If so, the shared ground is OK. The fault will certainly be at or near the lamp socket, as the truck’s rear wiring harness splits to both sides just beneath the tailgate. A fault forward of here would affect both taillights.

Brad Bergholdt is an automotive technology instructor at Evergreen Valley College in San Jose.