Primary reasons of MPG drop on a old GM car?

I'm driving a 1988 GM injector sedan. My MPG ratio had suddenly dropped after it was taken out of storage, having stayed in an outdoor garage for 2 years (California). MPG dropped close to 15%, which
bites.
After I took it out, I only flushed oil, and burnt several tanks of gas. Air filter was changed before the storage, spark plugs/wires - 6K miles ago.
I have no computer codes.
Would appreciate ideas.
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Was your gas treated before storage? You should have a professionnal clean your injectors, and the combustion chamber while you're at it. Have the plugs inspected, as well as all your sensors and thermostats.
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Beside the obvious tune-up procedures, I'd also check for any rolling resistance. Tires and brakes. Tires sitting for 6yrs is bad for them and brake parts can rust. Jack the car and see how easily the wheels turn.
Dave S(Texas)
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sticks open. s
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The 10% ethanol added to gasoline to replace a more toxic additive accounts for about 10% drop in mileage. My Impala that would 30-33 on the highway now gets 27-29 (if lucky). Roy

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