A quick answer would be 45 PSI. That's what I put in the tires on my mother's Prius.
Longer answer:
Go ahead and experiment! I have 55-60 PSI in my junkyard tires. They last me years and wear evenly. In my experience, some tires will wear out the center at higher pressures. Blizzaks are one example, they don't like high pressures. Others will do just fine. I don't know how much, if at all, I've reduced the tire's traction. I'm pretty sure I have more grip than the smaller original size tires at correct pressures. I'm still on the original shocks, springs and dry-rotted bushings at 239k miles. Ride quality and noise isn't great, but it's a gutted '91 Civic Wagon.
