1998 chevy Malibu revs high, losing gas

1998 chevy Malibu revs high, losing gas My 98 chevy Malibu has been reving higher and higher as th days goes by. What was just over 2000rpm has grown to just over 3000rpm staying steady at 2000 instead of 1500. I've also noticed my gas tank at full would get about 300 miles per tank but this time it only got 190 from full to empty. What could be causing this? Looking for all possible answers before I spend an assload at the mechanics.