Fuel economy in the real world

While fuel economy is predictable in the laboratory, it can vary by a few miles per gallon in the real world.

Many variables affect the gas mileage you can get from your vehicle. Where, when and how the vehicle is driven, the load and number of passengers it carries — even a driver's weight and temperament — affect fuel economy. So, fuel economy can change from day to day.

The two biggest factors affecting fuel economy are speed and acceleration. Slow, stop-and-go driving is a big drain on fuel, while high-speed driving has about the same impact. Frequent, rapid acceleration in traffic or from stop signs and traffic lights also saps fuel economy.

According to a General Motors study of one fleet of sport-utility vehicles, drivers who could and did drive with more steady and moderate speed and acceleration were able to increase their vehicle fuel economy by 12 miles per gallon (mpg). Drivers who didn't drive as moderately saw a decrease of 8 mpg.

Weather is out of your control, but you should know that driving on hot days or in extremely cold weather will also shave off miles per gallon, although not to the same extent as speed and acceleration. When you react to conditions by turning on headlights, windshield wipers, heated seats, rear-window defoggers, cabin heating and air conditioning — systems you need for safety and comfort — it has about the same effect as the temperature itself.

Be careful about how you choose to cut back on electrical load, though, because many electrical systems also impact safety. Always err on the side of safety. You never want to turn off headlights, windshield wipers or defoggers if conditions warrant their use.

The next-biggest impact on fuel economy comes from the weight, or load, that a vehicle carries. Load can come in the form of extra passengers or cargo, or from towing something such as a boat or trailer. The lighter the vehicle, the less energy it takes to move it along. The heavier the vehicle, the lower its fuel economy.

To get the best fuel economy, try to drive smoothly without rapid acceleration or stops. In heavy traffic, try to anticipate a stop ahead. Observe speed limits. Avoid unnecessary use of accessories, and don't overload your vehicle. It's also important to make sure your vehicle is properly maintained, taking special care that tires are properly inflated.

However, there are many conditions affecting fuel economy that are completely out of a driver's control. People who live in cold climates or hilly or mountainous regions will get lower fuel economy just because of those conditions.

With all this in mind, it becomes clear why individual driving experiences usually result in fuel economy measurements that differ with the official Environmental Protection Agency (EPA) rating — the one printed on the window sticker of a new car.

The EPA ratings
The main reason the EPA rating is required to be placed on new-vehicle stickers is for comparison shopping. The figures give a general idea of the fuel economy an average driver may actually get, but there may be wide variation depending on driving habits and conditions.

Fuel economy ratings are derived using a formula that actually calculates the average amount of fuel a car consumes per mile. The figure is then translated into the commonly used miles per gallon. Vehicles undergo two separate EPA test cycles that simulate city and highway driving. Engineers measure the emissions of hydrocarbons, carbon dioxide and carbon monoxide during the test cycles and apply those numbers to a formula that calculates the amount of fuel used. That calculation results in separate city and highway fuel economy ratings. Those two ratings are then combined into a "composite" mileage figure, with a little extra weight given to the city figure (55 percent) than the highway number (45 percent).

EPA ratings are good for comparison shopping because every vehicle is tested the same way under strictly defined laboratory conditions. Those lab conditions are repeated from vehicle to vehicle, so there is confidence that the only variable is vehicle performance. To repeat those lab conditions, engineers use sophisticated computers, sensing equipment and a dynamometer — a testing device that is programmed to run a vehicle through a specific set of road conditions without actually taking the vehicle on a road.

However, the tests are run with only one passenger — the driver — and are conducted without the heater or air conditioning running, so they do not reflect how people actually use their vehicles. There is a complicated formula for calculating the EPA fuel economy rating that tries to adjust for this fact, but it's still a formula rather than actual driving conditions.