Recently replaced headgasket - Now car is sluggish and gets 25% less MPG- Any Ideas?

botched job u may be loosing compression is my only thought there... also you could have a bad rewiring job... could be lots of stuff could you be missing something like a O2 sensor or something???

did you get the head looked at? its possible it could be cracked or warped

Start with a compression and/or leak down check on all cylinders to ensure that the combustion chamber is sealing.

Start with basics check compression in all four cylinders the proper way to do a comp check is with throtle wide open and pull the ECM fuse so you dont flood your Eng get 4 strokes out of every cylinder (you feel the comp tester pulse 4 times while holding it)

it is ok to have 10-15 psi difference anything higher is not good. Also most importantly you may be off a tooth on the camshaft check and make sure that you timing marks are aligned crankshaft and cam are aligned to their respective marks sometimes they move at initial start up I've had it happen. the dead giveawy that camshaft isoff distributor is moved to max adj it doesnt set at same spot prior to job

but you would know that right away wouldnt you??? ive seen it too but it was immediately obvious... when we had immediate engine failure that is... lol...