Spring/Summer MPG

The AC draws the battery down, so the ICE will want to run more to maintain the charge. It's not an issue at highway speeds or other conditions where the ICE runs continuously, and some suggest that the aerodynamic benefit of keeping the windows closed offsets any potential AC-induced mileage reduction at highway speeds.

At the other end of the spectrum, a "cold" start on a hot day is perhaps the worst scenario. The hot weather helps the ICE reach full operating temperature quickly, so without the AC the ICE can cut off sooner and staff off longer than in cooler weather. But on a hot day the AC would work its hardest and seems to deplete the charge the most, causing the ICE to light to recharge the battery when it otherwise wouldn't need to. I made the mistake once during the peak of last summer's heat of turning it on while making a couple of stops (without shutting down) near my office after work. I said, "Heck with fuel economy, I'm gonna cool off." That was the last time. My first five minute segment was lower than most I've seen in the winter. The AC just kept sucking juice out of the battery, and I didn't think the ICE would ever cut off.

My approach for this summer will be to try to avoid the AC at (nearly) all costs on startup, be comfortable on the highway, and try to balance comfort with fuel economy the rest of the time. I'll be doing more pulsing and gliding this summer than last (now that I've gotten pretty good at it), so it will be interesting to see how much the AC shortens the ICE-off glides.

 

While I agree with Jim, keep in mind that the battery is in the cabin. A hot battery isn't as efficient as a cool battery. I'll probably set my temp to about 77-80 and leave it. Last summer I set it at 75.

 

Just an FYI about the effect of warmer weather --

We bought our Prius last October and the weather was already turning colder. Our mpg started around 48-49 for each tank, then as the average temperature dropped it went down to 42-44. I even had one tank at 40.7 during a very cold stretch with snow.

Last week we had some beautiful weather (for Wisconsin) and my mpg for the last tank hit 50. Even better, for the first 150 miles on this tank I am sitting at 58 mpg.

I can't wait to see what I can really get when the weather is consistently warm!

 

Well, the AC will kill the mileage if you just let it run all the time. What I've found out works pretty well ans doesn't kill the mileage nearly as much. When driving on the highway use the AC at all times unless you are bumper to bumper. As we all know the AC kills when you are using local streets. That's why whenever you are at a stoplight just turn the AC off. When you start going again just turn the AC back on.