Oak Ridge answers A/C-on fuel use questions

The lab runs fuel-economy tests with A/C on maximum vs. A/C “comfort level,” A/C-off and with windows open and closed. Some results are surprising, including an unexpected “crossover” point in testing on a Toyota Corolla.

Which is more fuel efficient: rolling down the windows or keeping them closed with the air conditioning turned on? And how much fuel do you really save vs. windows closed with A/C off and perhaps the HVAC in vent with low blower speed?

There have been many studies on fuel consumption during U.S. EPA driving cycles, and a drive cycle (AC17) that specifically will target the issue from an official standpoint is forthcoming. However, Oak Ridge National Laboratory's (ORNL) Fuels, Engines, and Emissions Research Center couldn't find the answers to these typical “motorist's questions.” So it tackled them from what it considered a motorist's perspective-that is, it sought to answer the questions for a steady highway cruise in summer weather. A report on its results was given at the SAE 2013 World Congress by Shean Huff, a member of the research team.