He's being an ass.
As far as calculating the mileage i think you got it kind of wrong.... fill up the car, reset trip meter....then when you go to fill it up again take the mileage and divide it by the number of gallons you put in....like 10.6 or w/e...just round up or down.
As far as the bad gas mileage....tire pressure will def make you get a little worse mpg's....also that filter prob should be replaced...i mean it's not horrible but i would. Also, bad spark plugs and O2 sensors could cause it. How are your driving habits? Do you really get on it a lot? If so you might want to look into getting an intake....it will help a little with gas/performance. A full exhaust will really help....i get about 27mpg city/highway in the summer and 32 highway (with cruise on) with a 3500. So there's no way you should be getting that low of mpg.
Good luck!
Gotta love cars...if aint one thing it's the other.