Are they both Regal LS's or Regal GS's? If it is a 2002 GS and a 2000 LS, that is to be expected. The GS model is supercharged and will use more gas, especially in city driving (under boost more fuel will be consumed). The only major engine change (I know of) between 2000 and 2002 GS's is the removal of the "Performance Shift" button in lieu of the PCM activating the feature under "sport" driving.