1998 Ford Taurus fuel economy

1998 Ford Taurus mileage 110,000. I am a fellow licensed mechanic, up here in Alberta. This car was given to me with blown head gaskets. I replaced them and new head bolts, all plugs, wires, serpentine belt, put in new Synthetic oil like previous owner had since new. It does not use a drop of oil and runs like a charm. It had the engine service light on when I first restarted it, probably from antifreeze on the O2 sensor. Replaced them and light went out. The previous owner claimed it only got 20 MPG since new. I have driven it through three tanks of gas now, and he is right. The last tank had a new air filter in it but no difference. It has lots of power. I believe this car would beat my 68 Mustang with a 302 auto in a race. What do I look for to fix this mileage problem? It has no codes at all and I changed everything except the fuel filter as the previous owner changed it a month ago.