All Info About Auto Repairs
Your One Stop Source For All The Information You Need For Your Vehicles.

Understanding Fuel Economy Ratings

    What do those numbers on the window sticker mean and how do they arrive at them?

» Understanding Fuel Economy Ratings
» Factors That Affect Fuel Economy
» More Factors That Affect Fuel Economy
» How The Numbers Are Determined

How the numbers are determined...

Your fuel economy (miles per gallon or liters per 100 kilometers) can vary depending on how your car is driven. Several vehicles like yours have been driven through a standard test, and their actual fuel economy was recorded. These readings were adjusted and printed on the fuel economy window sticker that was attached to your new car when it was delivered and in the Gas Mileage Guide that is available from your dealership.

The fuel economy estimates are based on results of tests required by the U.S. Environmental Protection Agency (EPA). These tests are used to certify that vehicles meet the federal emissions and fuel economy standards. Manufacturers tests prototypes of new vehicles and submit the results to the EPA. The EPA then confirms the accuracy of the figures provided by the manufacturer. A professional driver under controlled laboratory conditions drives the vehicles on an instrument similar to a treadmill.

All of these tests are done with the air conditioning off and all other driver-controlled electrical loads off. These procedures ensure that each vehicle is tested under identical conditions.

There are two different fuel economy estimates for each vehicle: one for city driving and one for highway driving. To develop these two estimates, separate tests are used to represent typical everyday city and rural driving.

The test used to determine the city fuel economy estimate simulates a 7.5-mile (12 km) stop-and-go trip with an average speed of 20 mph (32 km/h). The trip takes 23 minutes and has 18 stops. About one-fifth of the time is spent idling as in waiting at traffic lights or in rush-hour traffic. Two kinds of engine starts are used: a cold start, which is similar to starting a car in the morning after it has been parked all night, and a hot start, which is similar to restarting a vehicle after it has been warmed up, driven, and stopped for a short time.

The test used to determine the highway fuel economy estimate represents a mixture of "non-city" driving. Segments corresponding to different kinds of rural roads and interstate highways are included. The test simulates a 10-mile (17 km) trip with an average speed of 48 mph (77 km/h). The test is run from a hot start and has little idling time and no stops.

To assure that the fuel economy numbers are most useful for consumers, the EPA adjusts these laboratory test results to account for the difference between controlled laboratory conditions and actual driving on the road. The laboratory fuel economy results are adjusted downward to arrive at the estimates on the fuel economy window sticker and the Gas Mileage Guide. The city estimate is lowered by 10 percent and the highway estimate is lowered by 22 percent from the laboratory test results. Experience has proven that these adjustments make the mileage estimates correspond more closely to the actual fuel economy realized by the average driver.

Even though these figures are adjusted, they still represent what the average driver will get. Your fuel economy may be significantly higher or lower, depending on how, when, and where your vehicle is driven.

Here are some things you can do to increase fuel economy:

  • Combine errands into one trip.
  • Turn the engine off rather than letting it idle for more than a minute.
  • Have your car serviced as described in the maintenance booklet.
  • Keep tires inflated to recommended pressures.
  • Anticipate traffic stops.
Here are some things that will lower fuel economy:
  • Quick acceleration.
  • Traveling at higher speeds. Traveling at 65 mph instead of 55 mph lowers fuel economy by 15 percent.
  • Carrying unnecessary weight in the vehicle.
  • Revving the engine. This is not necessary for your vehicle.
  • Operating your vehicle with the suspension out of alignment or with the wheels and tires out of balance.
  • Use of electrical accessories that require high amperage when they are not needed.

Even things beyond your control, such as weather conditions, affect your fuel economy. Driving up steep hills, in rain or snow, and into a strong wind will lower fuel economy. In the lower left corner of the fuel economy window sticker for your vehicle is the range of city and highway fuel economy you can expect to get from your car. If you are doing everything to raise your fuel economy but are still not within this range, your vehicle may need service. Collect as much information about your fuel economy as you can find (miles driven, gallons of fuel used, etc.) and provide it to your dealership for their review and assistance.

» Part 1       » Part 2       » Part 3       » Part 4
Additional Information provided courtesy of and Warranty Direct
© 2000-2007 Vincent T. Ciulla

FREE Newsletter. Sign Up Now!

Help keep this site free.

Copyright (c)2006

Search All Info About

Related Articles