8) I drove from Omaha, NE to Glynco, GA and back. Total of 2630 miles. Outside temp. 80 and average speed the speed limit 70 mph. I had the A/C set on auto temp 74 degrees. I averaged 50.2 mpg. I used only 53.6 gallons of gas for this round trip.
It depends on how much your local weather, route and driving habits keep the AC compressor and ICE running.
I live in Phoenix where it gets pretty hot in the summer. Over the last couple of months, I was getting about 42-44 mpg on my daily work commute, which is mostly city driving. This past two weeks, the drive time temperature has been low enough that I have been driving with the AC off and windows down, and I have gotten 54-57 mpg driving the same route.
I have been working a lot of overtime, and the traffic is lighter after rush hour, which may account for part of the higher mileage. But I was getting 52-54 last spring.
For highway driving, the AC seems to make very little difference; probably because the ICE is running most of the time.
I have a 2002 classic Prius. This may not hold true for 2004 and newer models.
Those numbers are calculated using Wayne Brown's THS II Simulator on Palm. My personal impression is that there is a bigger impact than what is show, but I know that I'm very sensitive to the smallest decrease in mileage from what I'm used to or had anticipated.
I'm fortunate to be able to keep my Prius parked in a garage 90% of the time and can usually avoid AC use for all or most of my daily commute in all but the hotest of weather or when the family is with me. When I do use it I tend to set the temp to about 80 (give or take a little depending upon whether the sun is shining directly on me or not which tends to affect my comfort level). I find that I'm quite comfortable at that setting. Even with the wife and kids in the car I can usually get away with a setting of around 78 degrees and I avoid grumbling about it being too hot.
I believe the mpg difference in an aero-dynamic vehicle like the Prius btwn a closed car w/air on, & an open car (windows down full or part way, causing air drag) w/air off, is negligible.
What about a closed car with the air off ? The most significant emotion associated with owning the Prius, is joy... so enjoy.
The major mpg influence is speed. I drive the exact same 152 mile trip once a month; 99% freeway. I experimented & compared 3 recent trips. Everything the same except speed.
Trip 1 75-80mph=40mpg.
Trip 2 65mph=51mpg.
Trip 3 55mph=61mpg.
Trip 3 was more dangerous than I anticipated. Although I stayed in the far right lane all the way, drivers insisted on trying to "show me" I was "wrong". One young man, whose hat indicated he did not know the difference btwn forward and reverse, gave me the Rockefeller Salute.
I live in Missouri. It is normally hot. (I say "normally" because this past summer and fall have been unsually cool....). I use the AC, nonetheless, ALL the time. I read that opening the windows can reduce MPG.
I get very good MPG. I just have to watch the consumption screen-- and take the "lead foot" off the gas pedal-- and do the "tapping" bit on the gas pedal...... It works. Anal-- but it works......
So, using the AC does NOT appear to negatively impact your MPG!
So, don't sweat it. Turn on the air! Happy driving.