My truck must have a problem then cause I cant get that kind of mileage or they didnt install the update and told me they did...

 

I don't understand it. I have an 04, 5.4 4X4 supercab and with the Borla exhaust, airaid intake and Superchips 1714 tuner on high performance tune I was getting upper 12's mpg.

 

I was wondering if anyone knows if I add a duel exhaust system to my truck would I get improved gas mileage and/or horsepower? Another question I have is that if I get slightly larger tires would I suffer from worse gas mileage and would it affect my speedometer at all?