Most road and hybrid bike tires need inflation to 60 to 140 lbs. per square inch, or psi, while most mountain and cruiser bike tires take 30 to 65 psi. Check the pressure recommendation printed on the tire, if available. However, different pressures within the acceptable range will give different riding results, and a tire that's underinflated or overinflated for the conditions gets flats much more easily. Proper inflation helps you avoid pinch flats, tire punctures and sudden blowouts.
Size And Type
The diameter of a tire, which is the first measure of a tire's size, does not generally affect the recommended inflation pressure. However, the late professional bike mechanic and writer Sheldon Brown wrote that "tire width and pressure are inextricably linked." If you have two equivalent tires, except that one is wider, the wider tire will function optimally at a lower pressure than the narrower one. Tubular tires, also called "wire-on" or "sew-up" tires, are a rare type of tire used almost exclusively for competitions. Some tubular tires used on road bikes can take pressures up to 175 psi, well above the 140 psi or so maximum for a 700 x 17 mm standard clincher road tire.
Rider Weight Effect
The heavier the rider, the higher the tire pressure should be. For 2-inch-wide tires, MountainBikeRiding.com recommends 30 to 35 psi for a rider weighing 100 to 150 lbs., but 45 to 50 psi for a 200 to 225 lb. rider. However, expert bicyclists disagree even on this recommendation. Former "Bicycling" magazine engineering editor Frank Berto's calculations are much more conservative, recommending 20 to 25 psi for the lighter rider and about 30 psi for the heavier. For road bikes with 700 x 25 mm tires, Berto recommends about 55 to 75 psi for a rider weighing 100 to 150 lbs. with an additional 30 lbs. added for the bike, but about 100 to 120 psi for the rider who is 200 to 225 lbs.
Other Factors
Because the rear wheel always carries slightly more of the rider's weight than the front wheel, Brown recommends inflating the rear tire to 10 percent greater pressure. Tire inflation guidelines from expert riders and bicycling technical writers tend to be higher than the manufacturer's rated inflation range. Berto says many tires can easily take higher pressures without adverse effects, but the manufacturer lists a lower range to avoid liability for tire blowouts and accidents. However, exceed the recommendations only at your own risk.
Tip
Check that you haven't gone to extremes with your tire inflation by sitting on the bike. Have someone else look at the rear tire if you have trouble seeing it. If the rear tire does not bulge sideways slightly under your weight, the tire is overinflated. Deflate the tire by 5 lbs. per square inch, or psi, and try again. On the other hand, if the tire bulges out considerably and above the rim edge, inflate the tire. Adjust the front tire pressure accordingly.
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