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

Instant MPG Displays

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.