Outside magazine, March 1995
The easiest yet most profound way to change a bike's performance involves nothing more than a pump and a valve. A difference of as little as ten pounds per square inch of tire pressure can have a big impact on traction, rolling resistance, and reliability.
On a road bike, inflate to the pressure recommended on the sidewall unless your usual route is rife with potholes, uneven pavement, or cobblestones -- then go ten to 20 psi lower. Much less, and you run a high risk of flatting from a pinched tube when you hit a bump. If you exceed the sidewall recommendation, the tire may separate from the rim in midride, when heavy braking makes the air in the tube nice and hot.
The sidewalls of fat tires usually offer an astonishingly vague inflation guideline like "25 - 80 psi." Go near the upper extreme only if you want a rock-hard tire -- i.e., only if your mountain bike stays on the pavement. The lower extreme will give you sufficient traction and cushioning on the trail but may feel squirmy and leave you vulnerable to flats. For general off-road riding, start somewhere near the middle -- say, 40 psi -- and inflate or deflate from there until you find the best compromise.