How accurate are gas mileage monitors?

Car Gadgets Image Gallery An energy monitor displays a Prius' miles per gallon. See more pictures of car gadgets.
William Thomas Cain/Getty Images

Ever since the meteoric rise of gas prices in 2008, gas mileage has become a top priority. The focus turned not only to the Environmental Protection Agency-rated mileage of our cars, but also to how that mileage could be optimized. It was even an issue in the 2008 U.S. presidential race, when then-candidate Barack Obama suggested that Americans check their tires, since proper inflation can dramatically improve gas mileage.

There's a problem with calculating accurate gas mileage, though, and there always has been. Most of us do a simple calculation each time our gas tank hits empty: number of miles on our odometer divided by the number of gallons we can fit in our gas tank. In theory, this provides an accurate assessment of how many miles we can drive per gallon. In reality, the number we come up with is a mere estimate that fluctuates by the tank, and even by the day.

This is perfectly fine for coming up with a rough number, but things start to get hairy when you start trying to improve your gas mileage. How do you know if the techniques or gadgets you're using are working if a rough estimate is all you can get? If your car's mileage varies by about 10 percent based on driving conditions, how do you know if the improvement you see after, say, adding acetone to your gas tank is a result of the additive or of the weather?

Lots of factors affect your car's mileage. For instance, snowy weather requires headlights during the day, wipers and defrosting your front and rear windshields. All this activity uses gas. On the other hand, warm weather might mean activating the air conditioning, which also lowers your mileage. And then there's the fact that gasoline takes up less volume in the cold and expands in the heat, so the amount of gas you can fit in your tank isn't even constant.


­Weather, hills, road conditions and frequency of stoplights all affect how many miles you get per gallon at any given time, so manual calculation after you've gone through an entire tank doesn't do much for determining an increase or decrease in MPG. The solution to this problem is one that briefly popped up in the 1970s due to the gas crisis and then promptly lost favor when prices went back down. It's real-time energy feedback. The concept made a huge comeback when gas prices start averaging $4 a gallon.

In this article, we'll look at several approaches to instantaneous MPG feedback and find out if, and how, they work to improve your car's efficiency.

Let's start off by looking at a few approaches that don't work, one of which we've already mentioned here.

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?


Good Gas Mileage: Knowledge Is Power

After driving with a mileage monitor, you may notice that you fill up the tank a little less often.
After driving with a mileage monitor, you may notice that you fill up the tank a little less often.
Justin Sullivan/Getty Images

The accuracy of car energy monitors is difficult to pin down. You'll find some sources that say yes, they return accurate figures; others say they see some real discrepancies. For the purposes of calculating MPG, this is an issue. But for the purposes of calculating a change in MPG, it doesn't really matter that much. If your gauge says 25 miles per gallon one minute and 27 the next, you can be confident your mileage increased, even if the numbers aren't spot on.

­The amazing thing about these real-time monitors is that they do tend to increase gas mileage. They don't change anything in the engine or the gasoline. What they change is the mind-set of the driver. It's a matter of cause and effect: When you floor it after a stop sign and you see a number on a screen drop from 27 miles per gallon to 20, it's hard to ignore it. Real-time energy monitors show exactly how much gas you're wasting (and how much money you're throwing away) each time you slam on the brakes or cruise along at 85 mph instead of 65 mph. With that level of awareness, behavior changes. It changes so much it can even turn into a game to keep breaking your own mileage record. Prius owners have been known to get into heated competitions to get the most miles per gallon (sometimes called hypermiling), which occasionally results in mileage that far exceeds the Environmental Protection Agency-mileage rating for the car.

The energy monitors that come in the Prius, the Nissan Altima Hybrid and the Lexus RX Hybrid, among others, are only one way to get real-time feedback. You don't have to go out and buy a new car. If your car is gas-powered (as opposed to diesel) and was made after 1996, it has a port you can use to hook up an aftermarket monitor like the ScanGauge or the Kiwi. Both cost a couple hundred bucks and do pretty much the same thing in terms of energy feedback.

For more information on energy monitors, gas efficiency and related topics, look over the links on the next page.

Related HowStuffWorks Articles


  • Allen, Mike. "3 Gadgets That Really Work." Popular Mechanics. DIY Auto. December 2008.
  • Allen, Mike. "How to Monitor Your Fuel Economy in Real Time on the Road." Popular Mechanics. June 9, 2008.
  • Cefali, Anthony. "Every Drop Counts: Nissan to Equip Cars with Fuel Efficient ECO Pedals." Gas 2.0. Aug. 5, 2008.
  • Rosenwald, Michael S. "For Hybrid Drivers, Every Trip Is a Race for Fuel Efficiency." Washington Post. May 26, 2008.