Good words from Hipshot and murray, +1
In the meantime, pull a plug from each cylinder and see how they look. They can tell you a lot about how the engine is running. Based on what you have said, they will probably be very black. When you throttle up hard, do you see any black smoke from the exhaust?
Both of these symptoms would be indications of running too rich (along with your poor gas mileage).
If the plugs look good and no smoke from the exhaust, make sure your tire pressure is correct (low pressure will increase your drag and pull mileage down <not to mention possible tire failure from overheating>) and make sure you don't have a caliper piston stuck and causing a pad to drag, that can really drag you down.