And it's not necessary to read the speed of the rear wheel - many modern bikes has a hall sensor in the primary sprocket cover and a magnet on the sprocket.
I guess not my triumph since the speedo is analog. I'm looking at the rear sprocket to mount the magnet with a switch or sensor on the swingarm or chain guard. The front sprocket is another thought for the sensor though. It would shorten the wiring be solidly located and under cover. It could be doing 50-75 revs per second so a divider might be needed there too.
What will your display look like?