AC seems to take the biggest hit on economy when I'm accellerating. If I turn the AC off until I'm up to cruising speed, it seems to make less of a difference when I turn it on, but if I start my journey from home with the AC on the whole way, my trip average economy will be quite poor.
For example, on my 5 mile commute to work every day, under the best conditions (i.e. I hit green lights all the way, which is do-able), I can get over 30 MPG. That's on surface streets without using the crosstown Interstate expressway. I try to average 40-45 MPH. There's a lot of elevation change, but my home is at a higher elevation than work, so on average the trip is downhill. If I leave home with the AC on, about the best I can do is 22-23 MPG. If I do the first mile or so with the AC off and then turn it on after I've climbed the highest hill and am on the descent, it'll be more like 26-28 MPG.
I cannot match those numbers taking the same route home from work.
I'd have a lot more fun in my car if it didn't have the nag screen. I'd probably punch it a lot harder and get into the turbo boost more. Ignorance is bliss.
It's not worth it to take the Interstate cross-town expressway, even though it takes me straight to work, because the journey isn't long enough to average out the fuel lost by accellerating up to highway speed, just to exit off three miles later. I fare better taking the surface route.