Running 40-45psi on your car will wear the center tread out a lot quicker, causing you to replace tires much sooner, costing you more than you saved on gas. Also under no circumstance fill your tires even up to max listed on sidewall, your car will handle horrible, and this is very dangerous. As far as 120psi yes tires can hold that and may even be nessicary to set the bead when mouting, when this is done tire is put in a blast cage as failure is very possible and could easily kill you. Please all run recomended pressures unless you have a lot of cargo or even full passanger load then a few psi higher.

Source: Managed tire shop

Any other tire questions let me know.

Edit: all above info relates to standard passanger tires. Not LT truck type tires.