How accurate are gas mileage monitors?

Gas Mileage: The Great Gas Quest

Tracking your miles per gallon can help change driving habits.
Tracking your miles per gallon can help change driving habits.
William Thomas Cain/Getty Images

New gas-saving gimmicks are popping up all the time these days. Adding acetone, aka nail-polish remover, has been touted to increase mileage. It doesn't. Same goes for putting a magnet on your fuel line and installing a water-injector system in your engine [source: Allen]. Water injection really only increases the efficiency of a turbocharged or supercharged engine [source: Rally Cars].

Our driving styles, on the other hand, have a surprisingly acute effect on how many miles we get out of a gallon of gas. For instance, accelerate too quickly and mileage decreases, because the injector has to squirt more gasoline into the engine to achieve a speed quickly than to achieve it slowly.

Incidentally, accelerating too slowly is also bad for MPG, since it keeps your car in a lower gear for too long. Lower gears are less efficient than higher gears.

Other factors reducing gas mileage include sudden stops and high speeds.

Most of us are aware of these factors, but making a long-term change in how we drive requires more than awareness. It requires a lot of attention. And we're usually too busy with getting where we're going to remember to start and stop more gradually. Enter the real-time MPG monitor.

­You find these on lots of cars, including many hybrids and even a few nonhybrids. They come in a range of forms. BMWs have had a low-tech version of the energy gauge for many years, in the form of a vacuum gauge situated in the instrument panel. This simple device keeps track of the level of vacuum in the intake manifold, which moves gasoline and air from the carburetor to the engine's intake valves. The vacuum is highest when you're cruising in high gear and lowest when you're idling. The greater the manifold vacuum, the greater your gas mileage. A vacuum gauge gives constant feedback on manifold vacuum, so you know in real time exactly how your driving is affecting your fuel efficiency.

A much more high-tech approac­h to real-time monitoring has popped up recently, most famously in the Prius. The Prius Energy Monitor is an LCD screen immediately to the right of the driver that keeps track of some great data, including whether the engine is drawing power from gas or from the battery and, of course, the car's MPG at any given moment.

But are these gauges accurate? And do they even need to be?