I had the same problem with a 96 Mystique GS V6 auto, then the water pump blew and the mileage went back to normal after the new pump was put in. FYI when the car was new, highway mileage was 36-40 MPG (imperial gallons, the highway were I live in Canada has a 110 km/hr speed limit), then dropped to high 20s. After the water pump was replaced, it went up to 34-36. Good luck. (here is my original post) Orginal Post

1996 Mystique GS, pacific green, stock. V6, ABS, traction control, alloys and steel rims, car seats for two, now SOLD. 2004 Toyota Sienna and 2002 Nissan Pathfinder (the new work truck).