01 Sunfire gas mileage Drop
 

I recently changed the head gasket and all other gaskets removed during the process on a 2001 sunfireand after i replaced them the motor runs fine and theres no more leaks but my gas mileage dropped horribly. The same thing happend after i changed the head gaskets on a 95 grand prix. i just wanted some possible ideas on why this happend. please andThank you