poor fuel economy

1991 Olds '98, 3800 Vin L engine, 249,000km, rebuilt transmission
Car consistently gets abyssmal fuel economy (last fillup was 50 litres for 341 km of city and highway driving)
I know this vehicle should be doing better as we have a '94 '88 with the exact same engine and transmission, and I have wrung 11km per litre of fuel out of it on a good day on the highway.
Parts I have changed in an attempt to improve this (with minimal effect) include: plugs, wires, o2 sensor, air filter, fuel filter, PCV valve, thermostat, ignition coil packs (to remedy an unrelated intermittent poor starting problem), catalytic converter has also recently been changed
Can anyone suggest where on earth my fuel is going? I cant smell any fuel leaks or see any wet spots under the car. The engine still runs quite well considering the amount of mileage on it and still accelerates briskly when the pedal is mashed.
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Try scanning it and looking at sensor inputs and fuel trim. You cannot just replace parts on a spfi engine and expect to stumble on a problem.

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Why not check the slack in the timing chain? That can hurt your mileage severely and could still run OK.
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