Originally Posted by KLCarch

Ever since I saw the 'mythbusters' guys do an AC vs windows test I've wanted to do one with my hybrid. They ran 2 expeditions around a track at 45mph: one with all the windows wide open, and one with AC and closed windows. the windows open car got better mileage. But, they weren't travelling at real highway speeds, so I've been wondering how much open windows really hurt.

So this past saturday I had the oportunity to do a highway run where it was fairly flat (unlike where I live). I had to pick my kids up in Corning, NY. The highway there- Rt 17 runs for 50 miles between Binghamton & Elmira pretty flat. there are some slight grade ups & downs, but not much.
I ran 3 rounds of each venting option- driving at mainly 60mph- but varying slightly between 55-64mph with the grade. I figure 3 rounds each way would average out grade & wind variations. roads were dry, temp was 85. rt 17 is a divided 4 lane in this area (2 each way). speed limit was 65; I was passed, but traffic was light, and mostly going the limit, not above. going west, I had a suburban about 10 car lengths in front of me as a windbreak. he was travelling at 60mph.
3 venting options: AC w/ windows closed; no AC- windows closed; no AC diagonal windows- 1 each side of car- open 6"

Driving West - 2 adult passengers:
AC no windows: 56 52.6 56.7 avg: 55.1

no AC/ no windows: (hot!) 64.7 62.8 59.4 avg: 62.3

no AC/ 2 windows venting 65.1 55.6 55.6 avg: 58.7

Driving East - 2 adults, 2 kids:
AC/ no windows: 54.1 54.1 52 avg: 53.4

no AC/ no windows: 63.2 55.6 57.9 avg: 58.9

no AC/ 2 windows venting: 57.1 60.0 58.6 avg: 58.5

overall mpg averages:

AC/ windows closed: 54.25mpg
no AC/ windows closed: 60.6mpg
no AC/ 2 windows venting 6": 58.6mpg

thus confirming the mythbusting theory to me: AC takes alot of energy & is less efficient- at least with these small engined hybrids. I did these test on flat terrain, but I know with my usual driving over hilly roads, the car really works to go up a hill and run AC. I don't use AC much myself anyway- but I'm in the northeast & it doesn't get over 90 much here.
perhaps 40 years ago, when test were first run for fuel efficiency with AC, the bigger powerplants were more efficient with the windows shut and AC, as well as larger window opening volumes and a generally less aerodynamic form overall.

on a side note- this is my first full tank since the recall- I had my oil changed to mobil 1 as well & I got my best tank ever: 55mpg. I love this car- it just keeps getting better & better. I DREAD the return of really cold weather this winter!

Kate