Get the car current on maintenance. Spark Plugs, Plug Wires, Distrib. cap/rotor, Air Filter, PCV Valve, Fuel Filter, Valve Adjustment, Oil, Trans Fluid, Coolant, *Timing Belt/Water Pump*, Timing Adjustment, Idle Adjustment, etc. Once that's all done drive normal(under 4,000RPM) & see how the gas mileage is.
Gas mileage numbers are tested under PERFECT conditions, which is BS in real world.