Most people already know that in the summer, using air conditioning uses less fuel/energy than rolling down windows, especially at high speeds on the highway. The increased aerodynamic drag creates a higher resistance to motion than running HVAC with AC. But how much of a difference is it? I wanted to find out with a controlled experiment.
Test Setup
On a 37C/98F sunny summer day, I did two back to back tests with a pre-chilled cabin to 20C/68F, and then another loop with the driver window down about 2/3. The test cycle was a ~40km/25mi highway run that I drove at 110kmh/68mph. I used the built-in trip computer to record energy consumption. The tests were done back to pack, in the same hour, so weather and traffic conditions were as close as possible to being identical.
Test Results
- The AC run resulted in 157.2 Wh/km (252.9 Wh/mi) being spent. A very efficient result and not far off the efficiency this Model Y needs to hit the EPA rated range.
- The window down run resulted in 164.4 Wh/km (264.6 Wh/mi) energy consumption on the same 40km loop. This is a 4.5% increase in energy consumption.
Final Thoughts
I already suspected that AC was going to win, but it’s satisfying to see evidence backing it up. Had I rolled down more windows down, the energy consumption would very likely increase proportionally. And aerodynamic drag increases exponentially with speed, so the delta would be even greater at 120 or 130 kmh.
Electric cars are very likely to be more sensitive to increased aerodynamic drag because their drivetrains are a lot more efficient. Something like 80-90% of the energy in the battery goes toward vehicle propulsion, vs. ~35% efficiency of gas ICE cars. The majority of the energy in gasoline is going to waste heat. Gas cars are so relatively inefficient, that you’re less likely to notice additional loads.
I’ve also recently tested the Tesla HEPA filter’s ability to clean out PM2.5 pollutants from the cabin, you can watch that video below. So not only will running AC keep you cooler at a lower energy penalty, you will also enjoy cleaner air to breathe.