Does it help mpg, hurt mpg, or have no affect whatsoever on the Vue GL's performance?? I've been using it a lot more cause I think it helps heat up the cabin quicker, but may I'm an idiot for thinking that?

Anyone ever test this, or know for a fact which is better? Obviously you want it on if you got a tailpipe in your face, but otherwise, what's the general opinion for those who use their heater or air conditiong?