Hey guys, don't argue, don't assume, and don't guess.
Go out and try it for yourself like I have, and like Ford has.
Drive in congested city traffic with lots of stop & go with Max. A/C on the whole time.
Then try it with it completely off the whole time.

A good driver will see a net of 40% lower MPG. It's a fact.
A not so good driver may see less of a drop, because poor habits are cutting into fuel economy already, so they have "less to lose" in the first place.

When I drive in the city on a "nice" day with A/C off, I'm often getting over 50 MPG.
This is due to lots of coasting with the engine off, and zero engine idle at stops. When Ford did their all NYC drive to see how far they could go on 1 tank ( over 600 miles ) they were getting 42 MPG in the city with A/C off.

With A/C on Max. driving the same roads on a mid-July day.... My MPG is about 30 MPG.
Right on what Ford says... about 40% drop in MPG with Max. A/C during city driving.