I can only go so far explaining. Start with the 2 wire sensor. Is it magnetic or find the little core or short dowel looking thing out of the plastic covering? If so, that makes the AC current. The star wheel off the sprocket nut or said design has 4 corners? So the sweep of the leading edge and trailing edge causes a wave pulse of current up the wire. The faster the 4 bumps pass the magnetic sensor, it heads up the wire off of a leg of an integrated chip (IC). Integrated chip is now on a motherboard. Pulse is sent thru something called 'truth tables.' In other words, a balance of checks and rechecks and bingo the speedo number, the clock number, the radio dial number. Find an EE to design the breadboard so you have a speedo to match the 2 wire sensor. and that's all she wrote on this end. Look up integrated chip, truth tables, electrical breadboard, and for each one, hit photos or the other name that is on the menu bar to see what you're up against designing a speedo from scratch.
If I were in meguyveer mode, I'd veer over to a simple tach that works off a spark plug wire. Cut a round face out with an eighth inch slit that carves the arch of the tach. This way I can see the needle. I'd write the gear number I'd have to be in with same numbers, then the mph number in a larger number. 65mph in top gear is where at the tach? 40 and 25mph is all I need to fill in the rest.
