Originally Posted by

**supton**
I use a quick-hand formula (usually have Excel or a calculator handy):

kph = cadence * 60 * chainring / rear_cog * 2.1 / 1000

mph = 0.62 * kph

Boiling it down a bit:

mph = 0.07812 * cadence * chainring / rear cog

Bike size doesn't matter. 26" and 700c tires are nearly the same diameter, so the equation holds. 650c, I'd have to rework the math but it should be close enough for a workout.

However, as you tire, your BPM goes up for the same power output (or speed). Or, your speed will go down as you tire, if you hold the same heartrate.