If your fuel economy is excessively low, your vehicle may need to be serviced or repaired. You may want to contact your dealer or a reputable repair facility to check your vehicle. The following are some of the diagnostic checks a mechanic will typically conduct for poor fuel economy:

• 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

The size class for cars is based on interior passenger and cargo volumes as described below. The size class for trucks is defined by the gross vehicle weight rating (GVWR), which is the weight of the vehicle and its carrying capacity. Fuel economy regulations do not apply to heavy-duty vehicles, so they are not tested. See Which Vehicles Are Tested for more information on these vehicles.

*Gross Vehicle Weight Rating (GVWR) is calculated as truck weight plus carrying capacity.

Fueleconomy.gov's Tax Information Center provides tax incentive information for

• Hybrids
• Diesels
• Alternative Fuel Vehicles
• Electric Vehicles
• Plug-in Hybrids

The Energy Tax Act of 1978 established a Gas Guzzler Tax on the sale of new cars with particularly poor fuel economy to discourage the production of such vehicles—trucks are exempt. It is collected directly from the manufacturer rather than the buyer. The amount paid by the manufacturer is disclosed on the automobile's fuel economy label (the window sticker on new cars).

The fuel economy figures used to determine the Gas Guzzler Tax are different from the fuel economy values provided on this web site and in the Fuel Economy Guide. The tax does not depend on the actual on-the-road fuel economy, which may be more or less than the EPA published value for some drivers.

The greenhouse gas estimates presented here are full fuel-cycle estimates, combining the three major greenhouse gases emitted by motor vehicles:

• Carbon dioxide (CO₂)
• Nitrous oxide (NO_x)
• Methane (CH₄)

Full fuel-cycle estimates include all steps in the use of a fuel, from production and refining to distribution and final use. This gives a more complete picture of the climate change impacts of using a particular fuel.

Some greenhouse gases have greater impacts on climate than others. Scientists have developed relative global warming potential numbers for each gas that gives its impact on global climate relative to that of carbon dioxide. We use the global warming potentials developed by the Intergovernmental Panel on Climate Change to combine the three gases into tons of carbon dioxide equivalent. We believe this gives the most complete and accurate reflection of the impact of each vehicle's energy use on the global climate.

Numerous assumptions and calculations are necessary to estimate full fuel-cycle greenhouse gas emissions in carbon dioxide equivalents. So, estimates from different sources will vary. Our estimates are taken from the U.S. Department of Energy's GREET Model developed by Argonne National Laboratory.