Forty seems way high to me...tire shops are notorious for overinflating off road tires.
Best way to get your optimum PSI for everyday road use is via the chalk test. Link below...
How to Chalk Tires | eHow.com
Use the chalk that carpenters use in string lines, so it'll transfer well from your tires to the asphalt. Don't buy the permanent chalk...it'll say on the bottle.
Simplest method is to chalk a tire, then drive on it for a few miles and see how the chalk wears. If it wears just in the center, you're overinflated. Just on the edges of the tire, you're underinflated. Takes a little trial and error, but once you get it right you'll get maximum life outta your tires.