Speed & Horsepower Estimator

Here's a handsome little wheel. It's just over four inches in diameter and is made of a stiff and slightly shiny cardboard. The copyright says 1959 but this wheel is like new. I like the color and type on this one.

I'm not sure what this was really used for. I'm going to go out on a limb and say it's for estimating speed and horsepower. I guess my real question is: who would use this and why?