%#$@%$@ whats up with my mileage?

[292farmer]

ok so i recently bought for 135$ a 1989 dodge dakota 3.9Lv6 its a reg. cab 8ft bed 2wd
i believe has either std 3.21 or opt 3.55 gear has 23/75r15 and gets the same mpg as my 72 c25 292I6/4spd/with 5.13s! 14mpg.......i'd expect at least 17-18mpg
i did a tune up, changed tranny fluid, checked brakes for dragging, but cant break into the 15s.. what is wrong?! i bought this p/u for a fuel efficient DD so i can work on my 72 GMC truck.
the only thing thats maybe wrong is the smog pump has no belt on it(didnt think it'd matter)
are their taller tires i can swap on? any ideas or am i gonna have to get rid of this thing? thanks everyone sorry for such a long post/rant