1990 Celica GTS Liftback Poor Gas Mileage

That's a rather significant difference in MPG. First easy thing you can do is check/change the air filter. Next thing is fill up the tank again and check your mileage all over. Perhaps you made a mistake in your calculations with one of the numbers.

A lot of things can cause a significant decrease in fuel economy. How does the car run?

How long has it been since the car was serviced? When was the last time the spark plugs were changed? Does the car have an automatic or a manual transmission?

Possible causes are: clogged air filter - you should be able to shine a light on the filter and see light on the opposite side obstruction in the exhaust system transmission problem like slippage or not shifting properly - if it has an automatic transmission, it should be translucent red and not brown or black. vacuum leak - usually causes drivability problems worn or bad ignition system (spark plugs, high tension wires, distributor rotor, distributor cap) incorrect ignition timing dragging brakes bad coolant temperature sensor or wiring between the coolant temp sensor and computer

My fuel economy nearly doubled when I replaced my rusty tank.

Do you keep it full, can you smell gas?

Sudden, significant change is seldom (never?) normal.

Check the gas gauge before you turn off and after you start the engine each time. If there is a difference, you are losing gas other than using it.

Is there any new or strange noises?

Does the car seem sluggish (perhaps brake is stuck) or engine not as smooth as normal (miss firing)?

Take it to a trusted shop. Soon. Sudden change bodes ill.

sdb