My 1995 Buick Century did this when the thermostat failed and stuck wide-open. The engine would never heat up as hot as normal, and the heater would take forever to finally make warm air. It caused the engine to eat fuel like mad trying to warm itself up. Dropped from my regular 23mpg down to 13mpg. The only reason I noticed is that the temperature gauge was lower than normal. Changed the thermostat (took about 10 minutes) and the car heats right up properly and fuel mileage went back to normal.