97' Rodeo with only 13mpg?

I have a 97' Isuzu Rodeo that only gets 12 -13 mpg. Most other people I have talked to say that it should get about 17 - 18 mpg. What can I do to increase the gas milage? It has 88,000 miles and the catlitic converter has never been replaced, would this help? Someone also told me that you could have the intake suck up more or less gas by messing with the gas gauge sensors; my gas gauge is also about 1/4 tank lower than what is actualy in there all the time, is this a sighn of the previous owner messing it up? Please help.