Because the chain moves at the same speed over both sprockets......the rear sprocket must make 7/4 revolutions for every one revolution made by the front sprocket
So.....when the rider moves the front sprocket through 190 revs/min......the rear sprocket moves through 190 (7/4) = 332.5 revs/min......and the back wheel will rotate at the same rate
So....the linear speed of the bicycle in inches per minute =
2pi * 9 * 332.5 = about 18802.43 in/min
The speed in mph =
[2pi *9 * 332.5]inches per minute * 60 minutes in 1 hr / [63360 in/ mile] = about 17.27 mph

