Fuel

MPG Ratings: Fact or Fiction?

Here's why the posted fuel economy figures for new vehicles rarely match up with their real-world mileage

Fuel economy remains a top consideration among new-car buyers, but disgruntled motorists are beginning to raise complaints - and in some cases legal actions - against automakers because their mileage doesn't match up with their vehicles' "official" estimates.

Why are the Environmental Protection Agency's fuel economy estimates often off-base? It has a lot to do with the manner in which gasoline-powered cars are evaluated for their energy consumption. While it seems logical to determine a vehicle's fuel economy by simply filling up the tank, driving it on a given stretch of road or a test track for a set number of miles and basing the results on the number of gallons consumed, this is not how the experts do it.

In fact, tested vehicles don't reach the pavement at all. Rather, a car's fuel economy is measured in a laboratory on a machine called a dynamometer, which is like a treadmill for cars, using a standardized test that's mandated by federal law. Automakers actually do their own testing and submit the results to the EPA, which subsequently reviews and confirms the data.

A professional driver runs the vehicle through five separate standardized driving schedules, one each to simulate city traffic, highway cruising, driving at higher speeds, operating a car with the air conditioning on and in stop-and-go driving with lower ambient temperatures.

Throughout the test, a hose is connected to the vehicle's tailpipe that collects the engine's exhaust, and the amount of carbon present is used to calculate the volume of fuel used. The EPA claims this is more accurate than using a fuel-gauge to physically measure the amount of gasoline being burned. Still, a "fudge factor" of 0.7 is applied to the evaluation process to help bring the EPA's estimates closer to reality.

But out in the real world, a host of other physical and personal factors affect a vehicle's energy consumption. For starters, cars and trucks used for evaluation in the EPA's tests are broken in and are in top mechanical shape. Even relatively minor upkeep factors like having incorrect air pressure in the tires can cost an owner a few mpg.

Also, cars and trucks subjected to fuel economy testing are "driven" without a full complement of passengers, cargo and options aboard - all else being equal, the heavier a vehicle is, the more fuel an engine will need to burn in order to reach and maintain a set speed. What's more, the EPA says that small differences in manufacturing and assembling can cause minor disparities in fuel economy from one model to another.

Other physical factors like the particular blend of gasoline sold in a particular area at a given time of the year, trip length, traffic conditions, terrain, temperature and the weather all affect a car's mileage. Certain exterior accessories like roof racks can increase a vehicle's aerodynamic drag, and will in turn decrease a vehicle's mileage, especially at highway speeds. Heavy acceleration and braking, high-speed driving, excessive idling, towing and engaging four-wheel-drive will also drain a vehicle's gas tank at a higher-than-average rate.

While the EPA's fuel economy estimates may not be a completely accurate prediction of the mileage a motorist will register, they're still valid as a source of comparison for car shoppers, even if only on a relative basis. For example, if one model is estimated to get a third better fuel economy than another, it's reasonable to expect the latter will cost about a third more to keep the gas tank filled, all else being equal.

Despite the EPA's best efforts at estimating the fuel economy of each vehicle model, the end result remains, "your mileage may vary."