14 mpg sounds very low. You should be over 20mpg. If the car runs ok and the plugs look ok, would suspect a possible fuel leak or possibly leaking injector or cold start injector. You may have a hang up in the chassis (dragging brakes etc).
One other thing that comes to mind is the trip meter could be innaccurate. They do fail regularly. Verify this buy checking it vs the odometer. Let us know what happens.
1990 190E 2.6
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1982 Suzuki GS 750T