1990 Geo Storm fuel mileage

Your math! :)

You should be getting double that. To be that low, my first suspect would be a leaking fuel line, especially if the car is rusty. Most commonly you'll find wetness on the ground along the right side of the car, but I'm not sure where your fuel lines run. There are other places to look for leakage but you should be able to smell the gas.

Misfiring spark plugs will cause low fuel mileage too, but then you wouldn't be saying the engine runs fine. Any sensor, injector, or fuel pressure regulator problem that causes poor fuel mileage is also going to cause a severe running problem. You'd also see a real lot of black smoke from the tail pipe.

Another thing that is often overlooked is a dragging brake. That would mimic driving up a steep hill constantly which would also greatly reduce fuel mileage. Stop on a slight incline, shift to neutral, then release the brakes. If the car doesn't creep downhill on its own, we'll have to diagnose a brake or parking brake problem.