CPO 2010 Prius LOW MPG After Major Repair

So I went in to get a tire rotation and the dealer noticed an oil leak. Since it's a CPO I told them go ahead and fix it.

Before the repair I was getting 42-44MPG after I got it back from the dealer I'm now getting 38.

Summary of Repair:

OIL LEAK AT INTERSECTION OF BLOCK, HEAD, AND TIMING COVER, RECOMMEND TO REPLACE HEADGASKET (INCLUDES TIMING COVER RESEAL)
-ONLY FRONT COVER RESEAL AUTHORIZED, RESEALED FRONT COVER ONLY.

TOYOTA EXTRA CARE AUTHORIZED TO REPLACE TIMING COVER GASKET ONLY AT THIS TIME. CUSTOMER TO HAVE CYLINDER HEAD GASKET REINSPECT AT NEXT OIL CHANGE SERVICE TO VERIFY OIL LEAK

CUSTOMER STATES THAT THERE IS A LOUD NOISE/WHIRRLING FROM THE HOOD WHEN THE A/C IS ON. INSPECT AND ADVISE.
INSPECTED FOR NOISE WITH A/C ON. COMPRESSOR DOES SOUND MUCH LOUDER THAN NORMAL,ALSO VIBRATES THE FRONT OF THE CAR SLIGHTLY. PERFORMED ACTIVE TEST, SPED UP TO MAX (7500RPM) AND NOISE IS MUCH WORSE. RECOMMEND TO REPLACE COMPRESSOR BEFORE CATASTROPHIC FAILURE OCCURS. ALSO REPLACE DRYER BAG/FILTER.

TOYOTA EXTRA CARE AUTHORIZED TO REPLACE A/C COMPRESSOR AT THIS TIME. EVACUATE, RECLAIM AND RECHARGE AC SYSTEM W/R134

So as you can see so much for buying a CPO I'm stuck with this car. Does anyone have any idea what could be the problem with this car.

 

Thanks OBJ, yes CPO =Certified Pre-Owned

I assumed they probably had the car going on and off during those repairs and I understand it could be low.

So I refilled up my tank and now I am halfway through it.