"gears" are Math. a Ratio.
the chainring (tooth count number ) turns the rear wheel ( count those teeth too )
Same number its a 1:1 Ratio.

front 2x as large as the rear , it's a 2:1 ratio..


then you have wheel size the Diameter..

and the having the diameter, another calculation gets you Circumference..

that, combined , can indicate how far down the road you get for every pedal rotation..





...