why does my car get 14 1/2 mpg

hi
I recently bought a 1989 dodge aries k car.
it runs good and was a great deal, because i bought it for 500 dollars and it has only 19000 miles on it.
but my problem is its only getting 14 and a half mpg.
its a 4 cylinder it should get atleast 25 mpg,
do i nead a new sensor of some kind
any info would greatly appreciated thanks.