The magic of pi. Knowing the above facts allows us to figure things like,  how fast will that go-cart travel? For example: If you had a go-cart with an engine capable of 3000 R.P.M's (revolutions per minute) with a drive train reduction of 2 to 1, with 10 inch diameter tires, then 3000 divided by 2 = 1500  (revolutions per minute after drive train reduction) Pi or 3.14159 times 10 = 31.4159 (inches traveled in one tire revolution) 1500 times 31.4159 = 47123.85 (Inches traveled in one minute) 47123.85 divided by 12 = 3926.9875 ( feet traveled in one minute) 3926.9875 times 60 = 235619.25 (feet traveled in one hour) 235619.25 divided by 5280 = 44.6248 (miles per hour). This is one example of using the weird sounding mathematical figure, Pi, to provide an answer to a simple question. Also:  Pi times (the radius times the radius) of a circle  produces the area inside a circle. For example, a  ten inch diameter circle has an area of: 5 times 5 = 25 (radius times radius)  25 times Pi or 3.14159 = 78.53975 inches