How Instant MPG Readout Works

City traffic
Is it possible that better driving habits -- even in a crowded urban area like this -- could really help you save money at the gas pump?
AP Photo/Andy Wong

Lately, it seems we're getting the same message from all sides: It's important for everyone to save energy. When it comes to our cars and trucks that means using less fuel. We know that using less gas helps clear the air, maintain the country's fuel supply and save a lot of money at the pump. But the problem is, sometimes it seems like the only steps you can really take to save on fuel are big steps, like buying a hybrid car, or trading in your gas-guzzling SUV for a gas-sipping compact car.

Of course, while those monumental steps promise big savings at the pump, they can also cost a lot up front, or mean a significant change in lifestyle. What a lot of people don't realize, however, is that simple changes in driving habits can drastically increase the fuel economy of the car they already own. Going easy on the accelerator, avoiding hard or sudden stops and other simple changes in driving behaviors can lead to fuel economy gains that quickly add up.


But how can you tell which driving maneuvers really help you save gas? The best way is with an instant miles per gallon (MPG) readout. An instant mpg readout, which is sometimes also called an instant fuel consumption display or instant fuel economy gauge, is a feature that lets you know how many miles per gallons your car or truck is getting right at that very moment. While many cars have fuel economy gauges that display the average fuel economy over time, instant mpg readout lets drivers see exactly how much gas stomping on the accelerator pedal actually burns.

Instant mpg readouts and other instant fuel economy displays have been a part of the green driving movement for some time. They're a popular tool among hypermilers -- drivers who constantly strive to get the most miles per gallon possible from their cars -- and many hybrid cars have an instant mpg display as standard equipment. Now, however, these handy little displays are going mainstream. Many automakers are offering them as standard equipment on new cars, and there are lots of aftermarket options available.

So, how does an instant fuel economy display work, anyway? Is there a way that you can install one if your car doesn't have one already? And maybe the most important question: How can you use instant mpg readout to save gas and money? Keep reading to find out.


Instant MPG Displays

Toyota Prius prototype
Toyota Motor Corp.'s new Prius prototype is shown during a reporters' test ride at the Fuji Speedway in Oyama, Japan on March 13, 2009.
AP Photo/Shizuo Kambayashi

Before you can effectively use an instant readout, it helps to know how it works and what it's telling you.

A car's instant mpg is actually much different than its fuel economy rating from the Environmental Protection Agency (EPA). The EPA estimates the fuel economy for all cars and light trucks sold in the United States, and the EPA numbers are what most people are thinking of when they talk about cars that have "good" fuel economy numbers.


The EPA's estimate has two parts: average city mileage and average highway mileage. The city mileage number is what a driver can expect to get in mixed driving situations. Mixed driving includes maneuvers like stopping and starting, idling a little in traffic and cruising at slower speeds. Highway fuel economy is what drivers can expect to get when cruising at highway speeds. Both numbers are based on EPA testing.

However, for most drivers, the EPA numbers give an incomplete picture. Because the EPA conducts its tests in a lab, the numbers provide a good overview of the fuel economy drivers can expect but they don't take an individual's driving style into account. Of course, this varies from driver to driver. Instant mpg readouts, on the other hand, give drivers up-to-the-minute information on how their driving habits are affecting their fuel economy.

Instant mpg displays give drivers the exact miles per gallon they're getting at that very moment. The devices have sensors that take into account the car's engine speed (how hard the engine is working), the fuel-flow rate, manifold pressure and throttle position. Those numbers are fed into the display's computer, which sorts and calculates the information into an instant mpg result, so a driver can see exactly how much fuel he or she is using at any given point.

There's an important distinction to make here. EPA fuel economy ratings are only an estimation of average fuel consumption for a given type of driving, and many cars come with an average fuel consumption display. That display shows the average fuel consumption over a given period of time. With that type of device you can see how much fuel, on average, your morning commute uses or how many miles per gallon you got on your last road trip. An instant mpg readout takes this idea one step further. Rather than the average miles per gallon your car uses over a set time, instant mpg readout lets you know the miles per gallon your car is getting at that very second.

Most experts agree that these devices are fairly accurate. Because they're calculating mileage information using actual engine data, not lab test results, an instant mpg device can be reasonably precise. In fact, the devices are seen as so accurate (and useful) that they're fast becoming standard equipment on many new cars and trucks.

That's good news for a lot of people that have bought (or will be buying) new vehicles, but what if you have an older car or truck and want to see your instant fuel economy numbers? Is there a good solution? Find out on the next page.


Where can you get an instant MPG readout?

2010 Ford Fusion hybrid
The 2010 Ford Fusion hybrid has an all-new instrument panel display that features a unique method of teaching owners how to drive in an eco-friendly manner.
AP Photo/Ric Francis


In many new cars, instant mpg readouts are standard equipment. This feature is particularly popular with manufacturers of hybrid cars -- mainly because instant mpg readouts clearly show just how good the fuel economy is in their latest product.


Some have even taken the hard data from instant mpg displays in a new and interesting direction. Ford, for example, has an all-new display for its 2010 Ford Fusion hybrid. The display features vines and leaves as part of the readout. The more green your driving is -- the less fuel you use -- the more lush the leaves and vines "grow." Similarly, other auto manufacturers have gauges that also use information from the instant mpg readout. When the driver is using a lot of fuel, the gauges turn red. When the driver is driving in an eco-friendly way -- using less fuel -- the gauges glow blue or green.

However, even if your car didn't come with these types of cool gauges or any form of instant mpg readout, you might still be able to get instant mpg information using an aftermarket mpg display.

In order to have one installed, your car needs to be a 1996 or later model. That's because in 1996, the government mandated that all cars sold in the U.S. must have an On-board Diagnostic system (OBD-II). While on-board diagnostics have technically been around since the late 1970s and early 1980s, it wasn't until 1996 that it was required that OBD-II systems report emissions information. Whether you know it or not, you probably have some experience with your car's OBD-II system. For instance, if your (1996 or later) car or truck's check engine light has ever come on, that's the car's OBD-II system saying there's a problem.

Since the car's OBD-II system is continuously collecting the data anyway, all an aftermarket instant mpg readout has to do is collect it, compute it and display the results for you. Most of these systems start at around $150. However, if you're able to effectively use it to save gas, it may eventually pay for itself.

Installing an aftermarket instant mpg readout isn't difficult. You simply have to connect the readout to your car's OBD-II port, and then run a wire to the display module in the vehicle's cabin. It's a fairly simply procedure, but if you're not very handy around cars or if you're nervous about proper installation, it may be best to have a qualified mechanic do the installation. Most automotive electronics stores that install car stereos and other aftermarket products should be able to do the installation as well.

Now that you know how an instant mpg display gets its information and what it all means, you can start applying that information to how you drive. Read the next page to find out how.


Using Instant MPG Readout

A hypermiler and her Honda Civic hybrid
Laurie With, shown by her Honda Civic hybrid in Sauk Rapids, Minn., is part of a dedicated group of people around the country calling themselves hypermilers who strive to get every bit of distance they can out of every drop of gas.
AP Photo/Jim Mone

Because the instant mpg readout gives you information that's, well, instant, you can see exactly how the way you drive impacts your fuel economy.

If your car has an instant mpg display, pay attention to it as you drive. After a while, you'll find that it's easy to refine your driving habits to achieve better mileage. In fact, one simple way to increase mileage is to lightly accelerate away from a stop instead of rapid acceleration. If you track the data the gauge displays when you use each of these methods, it'll be easy to see which technique consumes the least amount of fuel. You can track the data the readout displays as you brake, too. For example, accelerating toward a red light, then braking suddenly uses more gas than coasting toward the light and braking gently at the end. Minimizing brake use by maintaining a constant speed also saves gas because your car's engine is able to operate at a steady rate rather than having to constantly work hard to get your vehicle back up to speed after slowing or stopping.


As we mentioned earlier in this article, instant mpg readout is an important tool for hypermilers. Hypermilers are people who are intensely devoted to getting the best fuel economy possible. They use tricks like gentle acceleration, coasting and even shutting off their engines while they idle in traffic to maximize the number of miles they get per gallon. By relying on the data from their instant mpg readouts, hypermilers are able coach themselves to impressive fuel economy numbers. In fact, on a daily commute, for example, a hypermiler may know the exact places he or she should accelerate, brake and coast to get the best fuel economy for that particular trip.

While hypermilers tend to determine their success based on the average amount of fuel their car uses on a single trip or the distance they can travel on one gallon (3.8 liters) or one tank of fuel, the instant mpg readout gives them accurate, real-time feedback about how they're driving and how certain behaviors affect their fuel economy. So, instant mpg readout isn't the final standard that a hypermiler uses to judge driving performance, but it is an important tool he or she can use to train themselves to drive in the most economical way possible.

Up next, find out if you're up to the challenge of using an instant mpg readout on a daily basis.


Daily Use of Instant MPG Readout

Remember to watch the road, too.
Some hybrid drivers can become captivated by watching their instant mpg readout and forget to watch the road ahead.
AP Photo/Franka Bruns

You have to be careful when using instant mpg readout to coach yourself and to refine your driving style for the best fuel economy. In fact, some hypermilers have been criticized for practices that can be unsafe. For example, following closely behind another car on the highway (drafting) might save some fuel since your car's engine won't have to work as hard to combat wind resistance; however, following too close on the highway, especially at a high rate of speed, is a recipe for disaster. Similarly, since braking represents gas wasted through lost acceleration, some hypermilers might advocate not braking much before turns. Of course, taking a turn too fast also increases your chance of losing control of the car.

Another point to consider is that it's easy to get distracted by constantly staring at your instant mpg readout and not paying attention to the road in front of you. When coaching yourself with your instant mpg readout, try to keep your attention on the road ahead. When you first start working with it, you may find it easiest to focus on one maneuver at a time -- like accelerating from a stop -- to practice getting the best fuel economy while also paying attention to what's around you. Avoiding an accident is definitely worth losing a few miles per gallon.


Some aftermarket devices also let you look very closely at your overall fuel economy, even allowing you to download data and graph your fuel consumption. Others track the data themselves and enter it into their computers, comparing routes to see which way home from work will help them net the best fuel economy.

On the other side, however, are people who install or use their car's instant mpg readout for maybe a few weeks and then begin to ignore it, slowly going back to their old, less efficient manner of driving. This behavior actually defeats the purpose of the device. After all, the main benefit of instant mpg readout is that it can help you adjust your driving habits and assist you in getting better fuel economy from your vehicle. Not just for a few weeks, but hopefully for a lifetime of fuel-efficient driving. As long as you aren't so obsessed with good fuel economy that you're ignoring safety, instant mpg readout is a handy tool that can help you save fuel (and money) at the pump.

For more information about instant mpg readouts, hybrid cars and other related topics, follow the links on the next page.


Lots More Information

Related HowStuffWorks Articles

  • Blanco, Sebastian. "GM engineers speak up for installing MPG displays in every vehicle." Autoblog Green. May 18, 2008. (April 15, 2009)
  • Datko, Karen. "Make it a game with instant mpg." MSN Money. May 7, 2008. (April 15, 2009)
  • Dunleavy, MP. "Save gas: Learn from 'hypermilers.'" MSN Money. Aug. 6, 2008. (April 15, 2009)
  • Green Car Congress. "All New Nissan Models to Feature Fuel Efficiency Gauge." Aug 21, 2007. (April 15, 2009)
  • Tundra Headquarters. "Toyota To Make Instant Fuel Economy Gauge Standard." Jan. 10, 2008. (April 15, 2009)