Use Gmap-pedometer.com to measure a stretch of road, say 1000 m or if you like 880 yds, then time yourself running it.
Or, count how many cycles per minute she is pedalling (one full cycle is the downstroke of both left and right foot) and then count the teeth on the '2' and '6' gears.
Then compute your mph as follows:
((Front cog teeth/rear cog teeth) * wheel diameter (eg. 26" for a mountain bike) * cycles per minute * 60)/(12 * 5280)
which can be simplified to
((Front cog teeth/rear cog teeth) * cycles per minute)/40.6 for a mountain bike, or
((Front cog teeth/rear cog teeth) * cycles per minute)/39.1 for a road bike
Someone should double check my math.
Since bike wheels aren't exactly 26" or 27" in diameter these calculations give only an approximate result.
Last edited by cooker; 11-25-07 at 01:33 PM.