Only 40 miles on 1/2 tank of gas

1998 Buick Regal LS, 149,500 miles, 3800 series V6. Until recently this car has been very powerful, but sometimes under a load, like going up a hill, began to feel like it was missing a little. I changed the fuel filter a couple of weeks ago, but the gas mileage has been getting really bad. Tonight, I watched the gas gauge needle drop from just below the half-full mark all the way down to empty with the warning buzzer going off after only 40 miles! I could literally see the needle moving even when idling. It does not smell of gasoline and does not appear to have any gas leaks under the car or around the recently replaced fuel filter. The exhaust does not show signs of running too rich. Where could the gas have gone?