2011 Volkswagen Jetta TDI: Windows Vs. AC Test
June 20, 2011
On my drive from Los Angeles to San Francisco, I ran an informal fuel economy test and got some enthusiastic comments. Some of you wondered why I didn't do a windows-versus-air-conditioner test and, well, I had to agree. So heading south on Interstate 5 through the San Joaquin Valley, I took some readings that I found revealing.
Just to recap, there has been a debate over which is more fuel efficient -- air conditioner off and windows down, or windows up and air conditioner on? Windows up is more aerodynamic but running the air conditioner makes the engine work harder. Here's what I found.
I ran the test at 70 mph because I felt this was a typical speed drivers would choose. It was 94 degrees Fahrenheit with little wind and the road was dead flat. So, at 70 mph, with the windows up and the AC on, the instantaneous fuel economy gauge showed I was getting 41 mpg. (This was close to the 40 mpg I got in my test three days earlier.) With the front two windows down and the AC off I got 45 mpg -- a 9.7 percent improvement.
I was amazed that the improvement was so great. I was also impressed with how horrible it was to be in a really hot car at 70 mph with the windows open.
After a few miles of pondering the results, I decided to try the test with a different variable -- air conditioning off and all the windows down. This time I got 41 mpg, the exact amount I got with the windows up and the AC on. So, with two windows down it is a 9.7 percent improvement and with all the windows down the car is turned into a parachute.
My feeling has always been that the aerodynamics of cars is so different that you can't predict the results. So if you are curious about this, and you don't mind being ripped apart by wind in the name of science, try running this test yourself.
Philip Reed, Edmunds senior consumer advice editor @ 5,937 miles