An air pump, mounted on a wheel, which pumps ambient air into the tire chamber to automatically pump up the tire as the wheel rotates. The air pump generally comprises a cylinder and a piston, which relatively reciprocate to effect an intake stroke and a compression stroke. The intake stroke draws ambient air into a variable volume chamber. The pressurized air within the tire acts on the cylinder to effect the compression stroke, which forces the ambient air from the variable volume chamber into the tire chamber to pump up the tire.