There must be something strange going on... I just did a Columbus OH to Somerset PA and back trip, and despite having the A/C set to AUTO-69 the entire way, my mileage for the trip was just a hair over 50 (which really surprised me, given that it was my first time in mountains). Granted, it was almost nothing but freeway, but with outside temps in the 88-92 range just about the whole time, I'd have thought there would have been a bigger hit to my mpg than what I saw... which was pretty much nothing given that my Columbus/Cleveland trips have also been just a hair over 50mpg.

 

The key to MPG is how long at what speeds...the AC will take a toll but far less a toll if the battery is near fully charged and the temperature outside and inside are constant. Remember, the compressor will work far harder at the beginning of the drive to cool down the cabin than later on when it is just keeping the temperature constant. Once the interior is cool, the compressor doesn't have to produce as much cooling and the fan is the main driver of air. So if you are in the car for an hour in 90 degree outside temperatures, the first 20 minutes will be spent getting the car to optimal temperature the next 40 minutes keeping it that temperature. The first 20 might be intense but the lat 40 won't be much of a tax.

Gas mileage also will be better for a longer ride than shorter because the car is warmed up, the catylytic converter is warmed up and the car is using less energy to stay at a good temperature. When you amortize the cost of fuel needed over that time, it starts to diminish.

Short trips always kill MPG, especially when taxed by the A/C.

 

This is why it is always a good idea when you first come to the car after it being parked for a while to open some of the doors while you straigten out your things to let some of the air out. Next, drive for a while with the windows down (all or partway, depending on speed) and the circulation fan blowing on max to ventialte out all the heat. Finally, close the windows, turn on recirc, and begin AC. Switch AC onto max if going downhill, or braking to a stop -- use the momentum to cool the car rather than heat up the brakes. (It would have been so easy for Honda to make this automatic in "econ" mode...) Those tips seem to help me out, as I live on the backside of a steep hill that I must go up and over. When climbing I have windows down, when descending, I run max AC for "free" as I'd need to brake otherwise (charging is maxed). That means once I get to the bottom, AC barely needs to work at all to maintain cabin temperature for the remaineder of my (fairly short) highway commute.

This means I see almost no mileage difference between AC and no AC, since I must waste the same energy one way or another climbing my hill.

 

That's what I'm talking about! "Real" men don't use AC!

 

I have also noticed a big drop in MPG with the air running. It is supposed to be around 110 degrees here (Phoenix area) towards the end of this week. I have been slowly increasing the temp setting as it gets hotter (it is at 78 now). Has anyone noticed the sticker on the door jamb which says extreme heat is not good for the batteries ? I wonder what kind of effect a summer of 100+ temps everyday will do to the batteries.

 

I recently took a major hit while waiting in the carpool lane at my son's school, and doing nothing but short five-minute trips on my last 150 miles. I'm down to about 35 from my normal 42-45. I normally shut off the car while I'm in the carpool lane, but it's been so hot this past week I decided to keep it on. Now, I'm turning off the car and going into the school to wait for dismissal!

Aneil

 

When temps are above 90 I bring a 6-pack size cooler 1/2 full of water and ice, along with a 10" square dabbing towel.
Although not as nice as a cool, dry cabin but definetly is better than just taking the heat, and you get all the benefits of not using AC.
I might break my all time record on this tank (See my signiture below)

I bring my cooler when driving solo- but if family is along I'll manually cycle the AC.
I'll switch on the compressor while coasting or rolling down a hill, and never...even on pain of death....will I switch it on while accelerating or climbing a hill.

If "others" are riding along I'll use full auto set to a comfortable temp but hate that for the MPG hit. Autostop is more fun on unsuspecting people as well when the climate is set to full auto.

 

We have an '05 here in Phoenix and this is the second summer here and we've not had any problems. My '06 does see (along with everyone else) a significant drop in MPG. I was above the 50 mark and now can't get more than 45 or 46. That's around a 12% to 15% drop.

Do the Prius' fair the same?