For $10 - $20 the wacky world of consumer bicycle electronics will solve all speed/distance measuring issues with clinical accuracy....just program in the wheel rolling circumference and you are on your way. Put the read out in an inconspicuous place (steering stem?) and most will think it's a clock.
I put one of those little bicycle computers on my last garage-built bike in the early 2000s & they work very well on motorcycles. Sigma-Omega something was the brand. Just J-B Weld the trigger to a spoke, mount the sender carefully & you're all set.
I've mounted one on the Enfield, but have yet to calibrate it. I've got too many irons in the fire for my own good
The most important thing is measuring your tire circumference accurately. On the harley in 2002 or 3 I measured three full tire rotations on the driveway with chalk then divided that number by 3. From there it's a simple matter to program that number into the unit.