I have always found that on long trips cross the US that if I locked in at about 72mph the RPM would hover at 2700 and I typically got my best milage on those trips pushing 24mpg over a 800+ mile run. After the tune it look as if at 72mph the car now sits at 2500 rpm to hold that same speed. If that is what is happening and not a change in the guage then it could be that the car is using less fuel to create the power needed to hold speed . Over time/miles that is going to net some additional miles per tank. Keep in mind this won't really show up around town (atleast I am not seeing it) but if one is out on the highway locked in at a single speed for say a few hours then it should begin to show up in the mpg numbers.