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Our energy expert answered your questions about vehicle fuel efficiency. | Photo courtesy of ©

Our energy expert answered your questions about vehicle fuel efficiency. | Photo courtesy of ©

To help you save money by saving energy, we launched #AskEnergySaver -- an online series that gives you access to some of the Energy Department’s energy efficiency experts. As part of the series, Energy Department scientists, engineers and program managers will be answering your questions and sharing their advice on simple ways to lower energy costs and carbon emissions.

On-road vehicles account for nearly 60 percent of total U.S. oil consumption and more than a quarter of the nation’s greenhouse gas emissions, the major contributor to climate change. Taking small steps to improve vehicle fuel efficiency can go a long way to lowering the amount you pay at the pump and reducing your environmental impact. This is especially true now that the busy summer driving season has officially begun.

That’s why we’re answering your questions about vehicle fuel efficiency. We asked Dennis Smith, National Clean Cities Director and Technology Deployment Manager for the Energy Department’s Vehicle Technologies Office, to help tackle the questions that you sent to us via email and social media. Among his many other duties, Smith coordinates the nearly 100 designated Clean Cities coalitions in their efforts to expand the use of alternative fuels and other fuel efficiency technologies and practices across the country.

What's the verdict on driving with windows down versus using air conditioning (AC) when it comes to what we've commonly heard about air resistance? -- from Katie M. via email

Dennis Smith: Running your car's air conditioning is the main contributor to reduced fuel economy in hot weather. Its effect depends on a number of factors, such as the outside temperature, humidity and intensity of the sun. Under very hot conditions, AC use can reduce a conventional vehicle's fuel economy by more than 25 percent. The AC's effect on hybrids, plug-in hybrids and electric vehicles can be even larger on a percentage basis.

Driving with your windows down can also reduce fuel economy. Open windows increase aerodynamic drag (wind resistance), making your vehicle use more energy to push through the air. This effect is quite small at low speeds but increases at highway speeds.

Here are some tips for Improving fuel economy in hot weather:

  • Roll the windows down at lower speeds; use the AC at highway speeds.
  • Don't use the AC more than needed or set the temperature lower than needed.
  • Park in the shade or use a sunshade so that the cabin doesn't get as hot.
  • Drive with the windows open for a short time before using the AC. Letting hot air out of the cabin first will put less demand on the AC and help your vehicle cool faster.
  • Don't idle with the AC running before driving. Turn the AC on after you begin to drive or after airing out the cabin briefly. Most AC systems will cool the vehicle faster while driving.
  • Read your owner's manual. Most manuals explain how the AC system controls work and how to best use and maintain the AC system.
  • For plug-in hybrids and electric vehicles, pre-cooling the cabin while plugged into the charger can extend your vehicle's range. Also, using a warmer temperature setting for the AC will use less battery power.

Does keeping the air pressure in your tires at the correct amount really help fuel efficiency? --  from Claudine H. via Facebook

DS: You can improve your gas mileage by up to 3.3 percent by keeping your tires inflated to the proper pressure. Under-inflated tires can lower gas mileage by 0.3 percent for every 1 pound per square inch drop in pressure of all four tires. Properly inflated tires are safer and last longer.

If I take an average gasoline fueled car that gets 25 miles per gallon (or you pick the average mpg number) and compare it to an all-electric car, which one accounts for emission of more greenhouse gases in normal daily operation? -- from Jim S. via email

DS: For this question, we took the conventional assumption of 27.6 miles per gallon, the Corporate Average Fuel Economy standard. The chart below from the Alternative Fuels Data Center compares greenhouse gas emissions and fuel costs for a selection of common conventional and electric drive vehicles for a 100-mile trip.  

Emissions and Fuel Cost for a 100-Mile Trip

(compact sedans)

Greenhouse Gas Emissions
(pounds of CO2 equivalent)

Total Fuel Cost
(U.S. Dollars)



87 lb CO2



Hybrid Electric

57 lb CO2



Plug-in Hybrid Electric

62 lb CO2




54 lb CO2



Learn more about the sources and assumptions of this data.

Why measure electric vehicle efficiency in MPGe rather than kWh/mile (or kWh/km)? -- from @tedpercival via Twitter

DS: Most consumers are already familiar with the miles per gallon rating that has been used for many years and posted on new vehicle window stickers. The MPGe (miles per gallon of gasoline-equivalent) rating provides a way to easily compare the efficiency of advanced technology or alternative fuel vehicles with conventional vehicles. A gallon of gasoline-equivalent means the number of kilowatt-hours of electricity that is equal to the energy in a gallon of gasoline. According to the Environmental Protection Agency, a gallon of gasoline has the energy equivalent of 33.7 kilowatt-hours of electricity.