odometer reading

I have a 2002 buick century. Have been getting bad fuel mileage lately. Replaced plugs and wires . Went on a 225 mile trip was going to see if that helped mileage. Noticed that odometer was reading extra miles.i know the exat miles for trip.but it read 22 extra miles for the trip.does anyone have any suggestions where to start looking.any help will be apperciated.

Thanks charles