Miser question - better to run A/C and fan low, or vent and fan hi?

The fan takes relatively little power compared to the AC compressor. So, if you can tolerate operating the car without the AC that will save some power.

However, keep in mind that the traction battery needs relatively cool air from the passenger compartment to keep itself cool.

I am not aware that there's any particular bias towards running the engine just because the AC is on. The engine will run when the traction battery needs to be charged up. Classic owners will note that the engine will run whenever the AC compressor needs to run since the compressor is belt-driven off the crankshaft pulley.

The Prius heater core is supplemented by a couple of PTC electric heaters. However I recall that the two electric heaters consume around 600W. So, this is like 40% of a hair dryer trying to warm up the passenger compartment until the gas engine heats up.

 

When I've run higher blower speeds it has been my experience that the ICE won't shut off in idle conditions (stopped at lights.) So I tend to run the AC on blower setting #2 when I need AC. The ICE is more prone to shut down normally with this setting. Of course, the summer heat here isn't scorching so I can get away with the low blower setting.

 

I prefer the AC on manual & fan speed as low as possible for comfort, usually the 1st or 2nd button. If the ICE isn't up to temp then it will run at a stop. This will happen only at the 1st light or stop sign.

While you are looking to optimize FE also inflate the tires & block the grill.

3 pips to go on this tank which includes a trip to NYC--58mpg (MFD).

 

I've never seen a MPG hit from the A/C, but I've never seen temps. above about 36C either. I'm sure it's there, just not big enough to repeatedly measure.

I've proven the Mythbusters are not entirely correct. Both the anti-Prius (2001 Nissan Pathfinder LE AWD) and Pearl have significantly more drag with the side windows open than with the A/C on. Both vehicles state the same thing in the owners manual so I'm pretty confident it's correct after my testing. They didn't address using the "vent" rather than an open window.

The fan draws much less power than the A/C compressor, even the Prius one.

 

I did have a horrible AC hit when it was in the mid 90's here, very sticky, and I was stuck driving short stop and go with family in the car. I couldn't use my normal low blower setting. I've never seen a 5 minute bar that low...think it was about 15 mpg--way worse than midwinter at about 10 F when I still get 25 mpg for the first five minutes with grills blocked. A few miles of that and the average for the tank was shot even though it had about 300 miles on it @ ~53 mpg.

At very low speeds (mall traffic and lights) AC or heater operation can murder your mileage.

On the other hand, at interstate speeds the AC has little impact with my conservative blower settings. I ran the AC all the way home across the state at 72 mph and still managed 51 mpg, and that isn't the first time.