A road bike with 170 mm cranks: (The usual generic diameter value for road wheels is 680 mm, so the radius would be 340 mm.)
340 mm / 170 mm = 2.0. (The radius ratio)
2.0 X 53 / 19 = 5.58
This number is a pure ratio, the units cancel out. I call this a "gain ratio" (with thanks to Osman Isvan for suggesting this term.) What it means is that for every inch, or kilometer, or furlong the pedal travels in its orbit around the bottom bracket, the bicycle will travel 5.58 inches, or kilometers, or furlongs.