How is it that in adds for superchargers they claim increase in gas mileage?? Why is that? The supercharger just makes the engine more compressed so wouldnt it burn more gas.

 

The theory behind it, i think, is that if you have more power, you don't have to goose it as much. Notice that they say that under WOT it'll decrease your mileage, since you're making more power.
If you grannydrive, you're good to go, but you're not getting the supercharger to granndrive, are you?

 

Ah good point.