Just about everybody knows that it's hard to bike on a flat tire, and it's also true that a partially empty tire can mean a great deal of extra work while you pedal. For casual riders, more air pressure equals less work. For competitive riders, less work equals more speed and better performance.
How PSI Works
If a tire were filled to the same air pressure as the outside air -- approximately 14.5 PSI at sea level -- the tire wouldn't hold the bike up at all. For the tire to remain rigid, it must be inflated at a PSI significantly higher than that of the outside air. This causes the air inside the tire to push outward, giving the tire the "pumped up" feel that it needs.
Recommended PSI
The recommended PSI for your bike tires depends on the type of biking you do -- 100 PSI for road riding, for example. Mountain bikes have fatter tires and roll on rougher conditions, and should be inflated to 40 to 60 PSI. Hybrid bikes do well in the 60 to 70 range. Underinflated tires result in more rolling friction, and can burst or throw a rim if severely underinflated. Overinflation creates less friction -- and more speed -- but also gives a harsher ride, and makes tires vulnerable to puncture from sharp rocks and other road hazards.
Extra 20 PSI
Regardless of the recommended or actual air pressure in your tires, 20 more PSI reduces the rolling friction and increases your speed. Exactly how much is a complicated function with no easy answer. Some variables involved in this equation include your weight, the width of the tires, how much tread is on your tires, the terrain on which you ride, humidity and barometric pressure during your ride, design specs of your bike and what gear you're in. Professional cyclists engage the services of engineers to calculate the ideal PSI for one cyclist on one race on one given day. Lacking that, you need to "eyeball it," and fill tires to the maximum recommended amount -- then release air if the resulting ride is too bumpy.
Best Practices
Your specific tire pressure is less important than basic tire maintenance -- both in terms of personal performance and in terms of the lifespan of your bike and tires. Caring for your tires begins with owning a good pump with a reliable pressure gauge, and inflating your tires at least once per week. Stay within the recommended parameters. Aim for high pressure for flat, street rides and lower pressure for bumpy rides or rides that include road hazards.
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