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Why is my gas mileage lower than the EPA rating? What are the factors that affect fuel economy? And how do you determine your vehicle’s fuel economy?

The fuel economy ratings are an estimate.

The purpose of the fuel economy ratings is to provide consumers with estimates to use in comparing the gas mileage of different vehicles. It is not meant as a guarantee of the gas mileage each driver will achieve. Since no test can simulate all the possible conditions that affect fuel economy, such as climate, driver behavior, road condition, and car care habits, your actual mileage will always vary some from the estimates.

Fuel economy estimates are determined by laboratory testing.

Fuel economy is measured under controlled conditions in a laboratory using a standardized test procedure specified by federal law. The fuel economy tests (which EPA updated in 2006 to more accurately account for actual driving conditions that can lower fuel economy, such as high speed, aggressive driving, use of air conditioning, and cold temperature operation), are based on a detailed driving cycle (each car is driven in exactly the same way, for exactly the same distance, with the same second-by-second vehicle speeds, to the maximum extent possible). The tests also are conducted under controlled conditions, using gasoline or diesel fuel that meet very detailed specifications. The goal of the laboratory testing is to control as many of the factors that affect fuel economy as possible, to the maximum degree that it is feasible to do so. Manufacturers test their own vehicles following EPA's test procedures—usually pre-production prototypes—and report the results to EPA. EPA reviews the results and confirms about 10-15 percent of them through our own tests at the National Vehicle and Fuel Emissions Laboratory: http://www.epa.gov/nvfel/. Additional information about fuel economy testing is available on the following sites:

· FuelEconomy.gov site (Department of Energy and EPA): How Vehicles are tested: http://www.fueleconomy.gov/feg/how_tested.shtml

· Fuel Economy (EPA): Data and Testing page: http://www.epa.gov/fueleconomy/data.htm

Factors that affect Fuel Economy.

It is essential that EPA's fuel economy estimates continue to be derived from controlled, repeatable laboratory tests to enable a standardized or "level playing field," comparison between all vehicle models. Although EPA's fuel economy tests are designed to reflect typical driving conditions and driver behavior, several factors can significantly affect how many miles per gallon (mpg) your car gets: how and where you drive; vehicle condition and maintenance; fuel variations; vehicle variations; and engine break-in (this is described on Many Factors Affect MPG page): http://www.fueleconomy.gov/feg/factors.shtml. Because some vehicles are more sensitive to these factors than others, the impact of the changes will vary from vehicle to vehicle. The nature of current hybrid technology -- the addition of a battery as a second source of on-board power, sophisticated control systems, and sometimes a smaller engine -- makes a hybrid's fuel economy more sensitive to certain factors, such as colder weather and air conditioning use.

In May 2011, EPA and DOT’s National Highway Traffic Safety Administration (NHTSA) unveiled the redesigned fuel economy labels that provide the public with new information on vehicles’ fuel economy, energy use, fuel costs, and environmental impacts. For the first time, comparable fuel economy and environmental ratings will be available for all new vehicles, including advanced technology vehicles like electric cars. Consumers will be able to make comparisons—car by car —to ensure they have the best information to help save on fuel costs and reduce emissions.

Although EPA sets emission standards for pollutants that vehicle manufacturers have to meet, we do not set fuel economy standards for the vehicles currently on the market. Therefore, even if a vehicle does not achieve the EPA fuel economy rating, the manufacturers are not required to re-test or change the car for that specific model year. However, if a significant number of specific vehicle complaints are received, EPA has the option of performing confirmatory tests for the vehicle the following model year.

The U.S. Department of Transportation (DOT) does set fuel economy standards for cars and trucks through the Corporate Average Fuel Economy (CAFE) program. The standards are for the average fuel economy of the entire fleet of cars or trucks for a given model year as opposed to a specific vehicle or model. A penalty is assessed if a manufacturer's fleet does not meet the average standard and are enforced by DOT. For more information on CAFE visit DOT's CAFE web site at: www.nhtsa.gov/fuel-economy.

How to determine your vehicle’s fuel economy.

Given the above information, what is the best way to get realistic "city" and "highway" fuel economy numbers for your vehicle? There is no "official" answer to that question, but we can offer you these suggestions.

· Start with a full tank of gas. By "full," we mean filling up using a pump that allows you to lock a lever in place and take your hand off the hose and nozzle. Use the lowest rate of fuel delivery offered (most nozzles have two dispensing rate settings, and some have three). Do not top off -- when the nozzle first shuts off, do not dispense any more fuel.

· Record your odometer mileage (or, if you have a trip odometer, reset it to zero).

· Drive at least half the tank, preferably 3/4 to 7/8 of the tank, before refueling.

· During the time when you are measuring the fuel economy of your vehicle, try to avoid extended idling operation and the use of a remote starter.

· For "city" driving, a minimal amount of freeway/expressway driving can be included; for "highway" driving, measure when you will be taking longer trips that are predominantly freeway, with relatively little stop-and-go and infrequent engine off/engine restarts.

· Refuel at the same station, using the same pump and nozzle if possible. At the least, refill with the same brand and grade of gasoline.

· When refilling, do it as you did in step one -- automatic flow, lowest rate, no top-off.

To calculate the fuel economy in miles per gallon, take the number of miles driven and divide it by the total gallons purchased at refueling. For an even better estimate, take the average of two or three tanks in each case (city, highway). While there are minor variations over time, this allows you to monitor the vehicle's performance. A sudden drop in fuel economy not explained by the reasons noted below tells you that you may need to have maintenance done on your car.

To find out what you can do to improve the fuel economy of your car, please visit the following web site: www.fueleconomy.gov/feg/drive.shtml. Driving efficiently and keeping your car in shape will make a difference.

If your fuel economy is excessively low, your vehicle may need to be serviced or repaired. You may want to contact your dealer and ask them to check your vehicle and to determine that there are no outstanding defect reports or service bulletins which may affect fuel economy. The following are some of the diagnostic checks a mechanic will typically conduct for poor mpg:

· Perform "On-Board Diagnostic System Check" for diagnostic trouble codes (sensors or actuators which may be malfunctioning)

· Check for dragging brakes

· Check transmission shift patterns for slipping, use of all gears, lock-up operation

· Check ignition timing

· Check owner's driving habits including A/C usage, hard accelerations, carrying heavy loads

· Check emission control system

· Check vacuum hoses for leaks, kinks, proper routing

· Check tire pressure

· Check fuel type, quality, and alcohol content

· Check fuel pressure and fuel system for leaks

· Check air cleaner element

· Check coolant level

· Check ignition system for wet plugs, cracks, wear, improper gap, burned electrodes, or heavy deposits, cracking or improperly connected ignition wires

· Check for proper calibration of speedometer

· In extreme cases, there may be engine problems such as poor compression or faulty fuel injectors

If you are not satisfied with the outcome from the dealership, you can take your case to the Service Representative; the contact information should be listed in the owner’s manual. Be sure to document your observations thoroughly.

The Federal Citizen Information Center’s Consumer Action Web site provides information on warranties, recalls, and lemon laws at: www.consumeraction.gov/caw_automobiles_warranties_lemon.shtml and lists contact information for car manufacturers at: www.consumeraction.gov/carman.shtml. In addition, the Better Business Bureau’s Web site provides information on State lemon laws, which vary by state, at: www.bbb.org/us/auto-line/state-lemon-laws/.

External Links Disclaimer: Please be aware that links to non-EPA sites do not imply any official EPA endorsement. Furthermore, EPA does not accept any responsibility for the opinions, ideas, data or products presented at those locations, or guarantee the validity of the information provided. EPA does not guarantee the suitability of the information for any specific purpose.

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Topic Information
  • Topic #: 23002-32284
  • Date Created: 2/25/2011
  • Last Modified Since: 2/5/2014
  • Viewed: 1232

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