Fuel Mileage question

[peppers&eggs]

I just recently picked up my 3rd H2. My previous 2 used to get between 11-13 mpg. This one is reading right around 8 mpg. It was bone stock except for a tuner called G Force. $69 tuner. Doesn't look to good. Just taps into the MAF sensor. I disconnected it thinking that might be it but really no change. I know previous H2's I've owned I added exhaust, CAI and tuner. Could those 3 really make that big of a difference or is there something else I can look at or replace or change out? This is an 03 with 93K. Could that G Force have done something to the CPU and maybe get it flashed? Any thoughts are appreciated.