Could've just watched Mythbusters and come to same conclusion.
Mythbusters gave these results from their test:
Control, 35psi (manufacturer recomendation)
tyres at 10psi = 3.7% increase in consumption
tyres at 30psi = 1.2% increase in consumption
tyres at 40psi = 6.2% decrease in consumption
tyres at 60psi = 7.6% decrease in consumption
MythBusters: Tire Pressure Test : Video : Discovery Channel
Here is the test done at Ecomodder.com
Experiment: coast down distances (rolling resistance) @ various tire pressures - Fuel Economy, Hypermiling, EcoModding News and Forum - EcoModder.com
PSI/Distance in feet
20 / 479.3
25 / 524.8
30 / 621.0
35 / 621.0
40 / 639.6
45 / 687.5
50 / 702.0
55 / 699.3
60 / 702.0
Not going to be unsafe and 'overinflate' is a misnomer. Tire is rated for 60psi so anything under won't be overinflating. Just exceeding what the manufacturer suggest for optimum ride comfort. You want to bounce around but get more mpg, then you've got plenty of room to play with. Go past what tire is rated for, then we are talking overinflating.
Props for experimenting though.