or go to a bike store and get a sigma speedo, you can calibrate exactly to your tire size and have a perfect reading of MPH.. not some Italian idea of speed.. most of them have a trip meter and odometer for holding real mileage log/.. some you can set that to what is actual on your bike.. don't get the wireless because all that spinning stuff can create radio interference.. and don't get the cheap Bell from Wall Mart because it can't keep up over about 50 mph.. the good ones can clock into the triple digits. bonus! they have a clock, average speed calculation, trip time, top speed etc...
I hardly had to think, much less write down any thing and had it all there at the end of the trip..see end of video for stats..