No problem! The overall diameter of a IRC 140/70-17 tire is 626 mm according to IRC's website http://irc-tire.com/mce/tires/rx-01.html.
Thus, the bike moves forward 1966.6mm or 1.966 meters per tire revolution.
That is 508.6 tire revolutions per kilometer.
508.6 X 42/14 = 1525.8 countershaft revolutions per km X 4 = 6103 sensor signals per km. 5696 pulses per km if you use a 15 tooth sprocket.
If you live in the US, the rear tire makes 818 revolutions per mile and that works out to 9816 pulses per mile, 9162 pulses per mile with a 15 tooth sprocket.