MPG and running the air con

On a Prius... yes.
On a 430 -500+ horsepower diesel pulling upwards of 75,000 lbs... no.

 

I can't say when it comes to a big truck, but in my Corvette I get 2-3mpg more running the AC vs having the windows down on the interstate.

I never tracked MPG doing this, but in an international 9400 the wind would beat the snot out of you, but in a prostar it was almost like the windows weren't even down.

 

The A/c will consume more fuel than if it were off. It takes energy to run the compressor and the alternator (the blowers consume a lot of electricity if they are up high). Also, if your going slow, like in traffic then it will cause your engine fan to come on more often as well, costing you even more fuel. How much fuel is the big question of the day. I get extremely good fuel mileage, but I do not go down the road suffering in the heat, so the answer by most I think would be NO, The A/C will NOT effect fuel mileage by any measurably large amount within a single day.

I have been running my A/C on high non-stop for several days now and last fuel stop was this...

really crappy, old 53-ft non aero trailer, bad recap tires on it, but only 3800 lbs in it. 55-60 mph.

entry#377
trip# 925507
ODO: 578799
GALLONS: 89.126
MILES: 866 miles
PUMP TOTAL: 347.5914
DATE: August 19, 2013
STATE: tx
MPG: 9.717
PPG: 3.9