Try the obvious things first.


1) Is the air filter dirty?
2) Does the throttle body need cleaning
3) Is the clutch slipping, or torque converter unlocking to easily
4) If the fuel mileage is also bad, it could be a a combination of the above, plus bad plugs, wires or a rich fuel condition caused by a faulty sensor.

I'd put my money on a dirty fuel and or intake system.