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Fuel Economy Myths and Misconceptions

There are several myths and misconceptions about fuel economy. The misconceptions listed below are based on user questions and feedback to and are listed in no particular order.


You have to drive a small car to get good fuel economy.

FACT: Advanced technologies like hybrid drivetrains, plug-in electric vehicles, diesel engines, direct fuel injection, turbocharging, advanced transmissions, low rolling resistance tires and aerodynamic designs are allowing standard-sized vehicles to be very fuel efficient. For the 2023 model year, many of the top ten most efficient vehicles are midsized or large cars.


Manual transmissions always get better fuel economy than automatics.

FACT: Advances in automatic transmissions have improved their efficiency to the point that the automatic version of a vehicle often gets the same or better fuel economy than the version with a manual transmission. For vehicles offered in both automatic and manual transmissions, consumers can easily compare fuel economy using our Find a Car feature.


It takes more fuel to start a vehicle than it does to let it idle.

FACT: Modern fuel-injected engines start very efficiently, especially when warmed up. Idling can use a quarter to a half gallon of fuel per hour, depending on your vehicle's engine size, costing you about 1 to 2 cents per minute. Turn off your engine when your vehicle is sitting still, except when you are waiting in traffic or waiting in a line where you would need to turn it on and off frequently.


Vehicles need to warm up before they can be driven.

FACT: Modern vehicles can be driven within seconds of being started, though the engine should not be subjected to extreme loads until it has reached its normal operating temperature. Plus, the quickest way to warm up a vehicle's engine is to drive it.


As a vehicle ages, its fuel economy decreases significantly.

FACT: A vehicle that is properly maintained will retain its efficiency for many years. The EPA tests vehicles with about 5,000 miles since a vehicle's fuel economy will typically continue to improve through the break-in period. Vehicles that are 10 or even 15 years old will experience little decrease in fuel economy if properly maintained.


Replacing your air filter will help your car run more efficiently.

FACT: This is true for older vehicles with carbureted engines, but modern fuel-injected engines have on-board computers that automatically adjust the fuel-air ratio to the proper level. Changing a dirty air filter won't increase your fuel economy, but it might improve your engine's performance.1,2,3


Aftermarket additives and devices can dramatically improve your fuel economy.

FACT: Excluding full conversions that meet all EPA certification standards, tests have shown that such devices and additives do not improve fuel economy and may damage your engine and/or increase your tailpipe emissions.


Using premium fuel improves fuel economy.

FACT: Unless your vehicle was specifically designed for premium fuel or knocks severely with regular fuel, you will probably experience no benefit from using premium fuel over regular under normal conditions. Under severe duty operation, such as towing or hauling heavy loads (especially in hot weather), higher octane fuel might improve performance and gas mileage and reduce carbon dioxide (CO2) emissions by a few percent. Consult your owner's manual to see whether premium is recommended and under what conditions (e.g., towing). (See Selecting the Right Octane Fuel for more information.


The EPA fuel economy estimates are a government guarantee on what fuel economy each vehicle will deliver.

FACT: The primary purpose of EPA fuel economy estimates is to provide consumers with a uniform, unbiased way of comparing the relative efficiency of vehicles. Even though the EPA's test procedures are designed to reflect real-world driving conditions, no single test can accurately model all driving styles and environments. Differing fuel blends will also affect fuel economy. The use of gasoline with 10% ethanol can decrease fuel economy by about 3% due to its lower energy density.


All vehicles are tested for fuel economy.

FACT: Some vehicles are exempt from fuel economy testing. These include motorcycles, passenger vehicles (such as SUVs and passenger vans) with a Gross Vehicle Weight Rating (GVWR) of 10,000 lbs or more, cargo vans with a GVWR over 8,500 lbs, and pickup trucks with a GVWR over 8,500 lbs and an interior bed length of 72 inches or more. Exceptions for vehicle models prior to model year 2011 are somewhat different (see Which Vehicles Are Tested for more information).


Gasoline will "go bad" after a few months unless you use a fuel stabilizer.

FACT: A study sponsored by the US Department of Energy has shown that, when stored in a sealed container at typical outdoor temperatures, gasoline will stay within the ATSM standards for gasoline sold in the US for at least a year.4 Aftermarket additives can increase oxidation stability, but they are not necessary for gasoline stored under typical conditions for less than 12 months. This is good news for plug-in hybrid owners that may not consume a full tank of gasoline for several months.

View Data Sources…
  1. Thomas, J., West, B., Huff, S. 2013. Effect of Air Filter Condition on Diesel Vehicle Fuel EconomyAdobe Acrobat Icon. SAE Technical Paper 2013-01-0311.
  2. Thomas, J., West, B., Huff, S., and Norman, K. 2012. Effect of Intake Air Filter Condition on Light-Duty Gasoline Vehicles. SAE Technical Paper 2012-01-1717.
  3. Norman, K., Huff, S., and West, B. 2009. Effect of Intake Air Filter Condition on Vehicle Fuel EconomyAdobe Acrobat Icon. ORNL/TM-2009/021. Oak Ridge National Laboratory.
  4. Sluder, C. and S. Davis. 2022. “Assessment of the Effectiveness of Three Aftermarket Gasoline Fuel Stabilizers in Preventing Gum Formation and Loss of Oxidation StabilityAdobe Acrobat Icon.” SAE Technical Paper 2022-01-0486, doi:10.4271/2022-01-0486.

This website is administered by Oak Ridge National Laboratory for the U.S. DOE and the U.S. EPA.