This question gets addressed in a couple different newbie guides, and previous conventional wisdom says the effect of a/c on gas mileage is negligible. With summer coming up I got curious and discovered a more exact answer:

A/C consumes 0.15 gallons per hour on low (**** selections 1-3)
Up to 0.25 gallons per hour on high (position 4)
So assuming a gas price of $2.50, that's about 38 cents per hour to keep your *** cool. This works out to exactly a 1 MPG deduction at highway speeds.

This test was done on my '04 with 132,000 mi on it, right after an a/c inspection verified everything was good.

Conclusion: it's not quite negligible, but measurably small. About 5% reduction in gas mileage if you normally average 20 MPG.

 

Awesome work FWB!! Thanks bro! I've always been kind of curious.

It's good to know man

 

Thanks for the findings man. Much appreciated