How reliable are fuel efficiency monitors?

Since gas prices surged dramatically in 2008, fuel economy has become a major concern. The focus shifted not just to the EPA-rated mileage of our cars, but also to ways of optimizing that mileage. This became a topic during the 2008 U.S. presidential election, with then-candidate Barack Obama recommending Americans check their tire pressure, as proper inflation can significantly boost fuel efficiency.

There’s an ongoing challenge with accurately calculating fuel economy, and it’s been a persistent issue. Most people rely on a basic formula: the number of miles shown on the odometer divided by how many gallons fill the tank. Ideally, this would give an accurate measure of miles per gallon. But in practice, the result is merely an estimate that can vary from tank to tank, or even from day to day.

This method works fine for rough estimates, but things get tricky when you try to optimize your gas mileage. How can you tell if the techniques or gadgets you're using are effective when all you have is an approximation? If your car’s efficiency fluctuates by about 10 percent depending on driving conditions, how can you determine whether the improvement after adding an additive like acetone is due to the substance or simply because of the weather?

Many variables impact your vehicle's fuel economy. For example, snowy weather demands the use of headlights, windshield wipers, and defrosting for both front and rear windows, all of which consume fuel. On the flip side, warm weather often calls for air conditioning, which also reduces mileage. Additionally, gasoline contracts in cold temperatures and expands in heat, meaning the amount of fuel your tank can hold isn't constant.

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Weather, inclines, road conditions, and the number of stoplights you encounter all influence your car's miles per gallon at any moment, so a manual calculation after using up a full tank doesn't provide much insight into fuel efficiency changes. The solution to this issue briefly emerged during the 1970s gas crisis and quickly faded as prices decreased. This solution is real-time energy feedback, which resurfaced when gas prices hit an average of $4 per gallon.

In this article, we'll explore various methods of providing real-time MPG feedback and examine whether and how they can enhance your car’s fuel efficiency.

Let’s begin by reviewing some methods that don’t work, including one we’ve already mentioned.

Fuel Efficiency: The Ultimate Fuel Challenge

New fuel-saving schemes are emerging constantly these days. Some suggest adding acetone, also known as nail polish remover, to your gas tank to boost mileage. It doesn't work. The same goes for attaching a magnet to your fuel line or setting up a water-injection system in your engine [source: Allen]. Water injection is only effective in turbocharged or supercharged engines [source: Rally Cars].

Our driving habits, however, have a surprisingly strong impact on how many miles we get from a gallon of fuel. For example, if you accelerate too quickly, fuel efficiency drops because the engine needs more gasoline to reach higher speeds quickly than it would if you accelerated gradually.

Interestingly, accelerating too slowly is also detrimental to fuel economy, as it forces your car to stay in a lower gear for an extended period. Lower gears are less efficient than higher gears.

Additional factors that reduce fuel efficiency include abrupt stops and driving at high speeds.

While most of us are aware of these factors, making lasting changes to how we drive requires more than just knowledge—it takes consistent focus. And with our busy schedules, we often forget to start and stop gradually. That's where the real-time MPG monitor comes in.

­These monitors are found in many vehicles, including numerous hybrids and even some non-hybrids. They come in various forms. BMWs, for example, have featured a low-tech energy gauge for years in the form of a vacuum gauge on the dashboard. This simple device measures the vacuum level in the intake manifold, which transports gasoline and air from the carburetor to the intake valves of the engine. The vacuum is highest when cruising in high gear and lowest while idling. A higher manifold vacuum means better fuel efficiency. The vacuum gauge provides constant feedback, so you always know in real-time how your driving impacts fuel economy.

A more sophisticated approach to real-time monitoring has recently emerged, most notably in the Prius. The Prius Energy Monitor features an LCD screen positioned to the right of the driver, providing useful data such as whether the engine is drawing power from gas or the battery and displaying the car's MPG at any given moment.

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

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Maximizing Fuel Efficiency: Knowledge Empowers You

The accuracy of vehicle energy monitors can be difficult to determine. Some sources claim they provide accurate readings, while others report noticeable discrepancies. While this can be a concern for calculating exact MPG, it’s less of an issue when measuring changes in MPG. If your gauge shows 25 miles per gallon one minute and 27 the next, you can confidently say your fuel efficiency improved, even if the figures aren’t completely precise.

­What’s truly remarkable about these real-time monitors is that they do tend to improve gas mileage. They don’t alter the engine or the fuel itself. What they do change is the driver's mindset. It’s all about cause and effect: When you accelerate quickly after a stop sign and see your MPG drop from 27 to 20 on a screen, it’s hard to ignore. These monitors make you aware of how much fuel you're wasting—and how much money you're losing—each time you speed or brake suddenly. This heightened awareness leads to behavioral changes. It even turns into a challenge to keep breaking your own mileage record. Prius drivers, for instance, often engage in friendly (and sometimes competitive) contests to achieve the highest miles per gallon (a practice known as hypermiling), often surpassing the car’s EPA mileage rating.

The energy monitors found in cars like the Prius, Nissan Altima Hybrid, and Lexus RX Hybrid are just one way to receive real-time feedback. You don’t need to buy a new car to get this information. If your vehicle is gas-powered (not diesel) and was built after 1996, it has a port that allows you to connect an aftermarket monitor such as the ScanGauge or Kiwi. Both devices cost a few hundred dollars and offer similar energy feedback capabilities.