Depends on tire make... my car came with Dunlops OEM with a stated MAX of 51 psi.

As for mileage improvement... I think its more psychological than reality. You did something you though would improve mileage and you drove accordingly and tahda... you got better FE. Try it for a month and tell me what improvement you made if any.

I did this experiment. I drove at +5 psi over OEM spec for 1 month and another month at 51psi (MAX). Well at first I thought I was getting better gas mileage at MAX but it turns out that I only gained about 10 miles per tank... which is nothing.

BUT... what did improve (the higher the PSI):

o Stopping.
o Cornering.
o Stopping in the rain.
o Traveling at high speeds (70 - 80 mph) in the rain.

So for those reason I increase psi... to about plus 10 over OEM.

Please note... that the higher you go the firmer the car feels so you will feel bumps more than normal.