If tire rating is 55psi I would run 60...But would never recommend you do that.
I have 44psi on my sidewall and run 55-60
Not sure of your background but your driving style is most important thing right now to MPG. Look up Wayne Gerdes at Clean MPG lots of teaching, here's a similar question on your car
Optimal Acceleration and Coast Speed for Scion tC | CleanMPG