How is Fuel Economy Tested?

You’ve likely noticed that a car’s fuel economy is one of its most important features in today’s car market. The unstable prices of gas and growing concerns about the environment have all contributed to this rise in importance. But how does a car’s MPG actually get measured, and can you trust the rating given to it? Read on to find out.

Auto manufacturers are legally required to post their vehicle’s fuel-economy ratings, as certified by the federal government agency, the Environmental Protection Agency. These ratings are generally posted on a new car’s window stickers in dealerships. The only vehicles that don’t need to post these ratings are those with gross-vehicle-weight ratings over 8,500 pounds.

fuel economy2That said, these EPA accredited, “official” ratings don’t always reflect a driver’s reality. Depending on what you drive, where you drive it, and how you drive in general, you can find some major differences in terms of fuel economy.

The most common complaint is that even the most cautious┬ádrivers are not getting as high of a fuel economy as they expected. This is largely due to the fact that new cars and trucks are evaluated for their energy consumption. While that would seem somewhat logical to figure out a vehicle’s fuel economy simply by filling up the tank and seeing how far you can get, driving it on a road or a test track for a set number of city or highway miles, refilling the tank, and dividing the number of miles driven by the number of gallons consumed, this is not how the “experts” see fit to measure a car’s MPG.

Cars tested for mileage don’t touch the pavement in any way. A car or truck’s fuel economy is instead measured under very particular, rigidly controlled circumstances that could only be maintained in a laboratory. This is mandated by federal law, but automakers actually do their own fuel economy testing and submit the results to the EPA. The EPA only reviews 10 to 15 percent of the ratings at the National Vehicles and Fuel Emissions Laboratory, after which it confirms most.

prepaHow does the EPA manage this process? It doesn’t drive the vehicle, that’s for sure. Instead, it tests the vehicle on a device called the dynamometer, which is basically a giant treadmill. While the engine and transmission drive the wheels, the vehicle never moves out of its position in whatever laboratory; instead, the rollers upon which the wheels are placed are what ends up moving. A professional driver runs the vehicle through two standard driving schedules; one is meant to mimic the strains of city driving and the other is supposed to mimic the stressors of highway driving.

The city program is supposed to mimic rush hour situations, whereas the highway program is meant to emulate rural and interstate freeway driving.

As a model undergoes the test, a hose is connected to the vehicle’s tailpipe that collects its engine’s exhaust. The amount of carbon present in what’s been emitted from the pipe is then measured to calculate the amount of fuel that was burned.


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