Summer in Colorado is not bad since it is a dry climate.

A/C use on the highway will be about 5% to 10% less MPG.
A/C use in the city depends if you put it on Max. or not.
If you put it on Recirc, and allow the A/C to cycle on and off at stoplights with the engine, then the hit on MPG is not too bad.

I think even in summer, you'll top 30 MPG in a nearly 4000lb vehicle, which is on par with the Mini!
-John