HELP! 96 T/A getting 10 MPG

I have a feeling the car is going back to the dealer but I figured I would post here first for some opinions. Its a 96 T/A with auto. Last week I got
it back from the dealer after they replaced the intake gaskets, timing cover gasket, Opti-spark, water pump, thermostat, crankcase sensor etc...(don't even ask about the tab).
Here is the problem....before the surgery I was getting 20 MPG, since (1 full tank) I am getting 10 MPG....maybe. Last night I drove it 60 miles round trip on the interstate at 60-70 MPH and it cost me 1/2 a tank. I got a grand total of 158 miles out of that tank of gas.
The car runs like a top.
Any ideas on why its so thirsty all of a sudden?
Thanks for any info or insight
Wes
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If the PCM was disconnected for more than 30 minutes, it went into stupid mode, and is currently adapting. id want to give it another tank and see what happens.
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