This measurement refers to the “pounds per square inch” of air pressure within the tire. Each tire manufacturer will list a range of appropriate PSIs; if the manufacturer says the tire should be inflated to between 55 and 70 PSI, we would list it in both the 40 to 60 PSI range and the 60 to 80 PSI range. It’s always best to inflate your tire within the range specified by the manufacturer, because the tire was designed to perform best within that range. Adding more air to the tire will not improve tire performance.
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Bicycle Tire, LLC "A Reference Resource & More.." 2009 
