I'm afraid it is not that simple.  While for most people using the cruise will improve gas mileage, it really does depend on the make/model of the vehicle and the driver's habits.

Using the A/C will most assuredly reduce your mpg.  It's simple physics.  More resistance on the engine means more energy (fuel) required to overcome that resistance.  But here again, make/model makes a huge difference.

My commute doesn't involve too many long unbroken stretches that enable a lot of cruise driving, so I have been forced to perfect my non-cruise driving skills....and now I can consistently get better mpg than what I could using cruise control, especially in hilly terrain.

Tips:
Pay attention to the road, not the cell phone.  You will be better able to predict braking/coasting stretches, as well as adjust acceleration for short spaces between stops.
It may sound silly, but using the same logic that pilots use seems to make a big improvement on the mpg.  And that is, accelerate to your maximum speed on a moderate stretch (up to 2 or 3 miles), then back off the gas just enough to SLOWLY lose speed until you need to brake.

My car has real time mpg data displayed on the console - so these techniques are proven.

Most of all be safe!

I notice Umbra admits cruise control is not more efficient in mountainous terrain.  I live in a "hilly" (for want of a better term) area and I would say the same goes for my terrain.  As Richard pointed out, humans have the ability to anticipate a hill and act accordingly; cruise control does not.  I can improve my mileage over cruise control by slowly increasing my speed going down a hill and then trading that momentum off for less gas consumption, going up the next hill.  Cruise control actually slows the car down, going down hill, and then has to use a lot of gas keeping a constant speed going up the hill.

Hi All,
    I agree that the effectiveness of cruise
control depends on the driver and the car, but
I would also like to offer my own experience
driving in mountains with a Honda Civic Hybrid
and using cruise control almost all the time.
It really does save a lot of gas, even over
mountain passes.  I average 51MPG, and I drive
Kenosha, Trout Creek and Poncha passes every
week on US 285 in Colorado.  My car is a manual,
which means I need to select the right gear for
the terrain.  With a tiny 1.3 liter engine I can
accelerate from 35 to 50 up a 7% grade using the
resume button on my cruise control.  I have found
that I need to do this in third gear, but that's
fine, the engine doesn't redline until 6000 rpm.
It's quite possible that my battries buffer the
hills and make cruise control more effective. and
having a manual transmission means I can inform
the car about the terrain ahead even with cruise
control.
            Cheers,
                  Susanna Gross

The issue addressed by Edmunds is not A/C on or A/C off, but rather A/C on AND windows up or A/C off AND windows down.  Having the windows down increases aerodynamic resistance, having A/C on increases engine resistance.  According to Edmunds, it's basically a wash.

On a side note, it sure would be nice to have a car with A/C...

Ubra needs to spend less time on Edmunds and more time on Ecomodder.com, gassavers.org, or cleanmpg.com.

A/C vs. windows down is going to vary by car and speed.  On my car, a/c is unquestionably less efficient than windows down, even at 75 mph. The previous posters comments on cruise are correct as well.  On flat terrain, cruise is great.  Once you get into uphill/downhill stretches, however, cruise controls preference for constant speed means it's going to increase engine load accordingly on uphill stretches to maintain that speed.  It's much more efficient to slow down a tad on the uphill and then try to gain it back on the downhill if possible.  I'm just scratching the surface here.....

Umbra needs to spend less time on Edmunds and more time on Ecomodder.com, gassavers.org, or cleanmpg.com.

A/C vs. windows down is going to vary by car and speed.  On my car, a/c is unquestionably less efficient than windows down, even at 75 mph. The previous posters comments on cruise are correct as well.  On flat terrain, cruise is great.  Once you get into uphill/downhill stretches, however, cruise controls preference for constant speed means it's going to increase engine load accordingly on uphill stretches to maintain that speed.  It's much more efficient to slow down a tad on the uphill and then try to gain it back on the downhill if possible.  I'm just scratching the surface here.....

The open window vs. AC feud has been going on since the wagon trains hit Oklahoma. In short, which is worse, the turbulence created by opening the window or the increased loading on the engine from the AC?

Most research seem to side with the AC being the lesser of two evils, but wait, there's more! As car have dramatically improved aerodynamically, opening the window can have a more profound effect, and the increased efficiency of the engines and AC units have also tipped the formula.

Our old '57 Chevys may be high in nostalgia, but they are a real drag, in a air resistance way. My Camry may lack drama, but it's slippery.

My solution? Around town, open the window to get what passes for fresh air these days. Hit the Highway? Go with the AC.

Bottom line: the color of your car has more effect than the above argument anyway. White cars are many degrees cooler than black ones. Heat in the winter is essentially free, from a gasoline mileage standpoint, so it doesn't matter. But in the summer, the AC is going to go into overdrive trying to cool off a dark colored car. So buy a light colored car, wear light clothing, and take the bus!