Fuel Consumption !

If you are sure you don't have any leaks at engine, lines or tank, then the gas has to be going out the exhaust pipe. It may be a weak spark getting to some or all cylinders, caused by low voltage into or out of Ignition-coil or sparkplug gap set too wide. Or it may be engine timing adjusted incorrectly.

Given what these weigh, unless you previously got much better milage 8 isn't too far out of line for city travel in this car. However I'd think more like 14-15 about normal. But I'd recheck my figures first - note the location of the gas needle with it at 1/2 tank or below (1/4 or 3/8 would be ideal), add 3-5 gallons of gas, note again needle position and odometer, and drive until the needle's at the low mark again. Maybe do it a couple times, and have the car level when you observe the needle. Idea is to try to eliminate some of the variables, and check when the tank float is actually floating and not submerged (it can only go so high in there). Every GM car I've been in the guage runs slowly until they get below 1/4 tank or so, then they drop like a stone.

If gas is going out your tailpipe, the car might have a tendency to backfire when it's hot as the gas burns in the exhaust instead of in the engine.

What shape is your transmission in? It slipping at all? Power steering, air conditioning on it? Brakes and parking brake working correctly, wheel bearings in good shape? making sure nothing is creating a drag. Gasoline of good quality and correct octane? Sometimes this will make a difference if your car isn't quite tuned properly.