Sentence:  These are MPH for Car speed RPM for Engine crankshaft speed R for Rear axle gear ratio W for Tire size Tire size can be determined in several ways but the one that is the easiest and as accurate as any is by <WSD>measuring</WSD> the effective radius of a wheel and tire assembly . 
Meaning of the word marked with <WSD> tags:  determine the measurements of something or somebody, take measurements of