Another possible implementation is to make a circuit based on the ubiquitous 555 timer chip. This is an adaptation of the standard 'astable' circuit, which produces an output wave that is more or less 50% 'high' and 50% 'low' - this is known as having a 50% duty cycle.
Again, the circuit was simulated in LTSPICE, and the circuit generated a ~5V square wave with a frequency of 295Hz. Again, expect this to change slightly due to the usual component tolerances. This equates to a speed of approximately 108mph for my K9800 speedo.
Some important notes on the schematic: first off, yes the diode is the 'correct' way round. It's a special type of diode called a Zener diode, which is designed to conduct in a controlled manner when placed in reverse. I'll spare you the electronics lecture for now (this information can always be Googled), but suffice to say, in this circuit, it acts to cap the voltage of the produced square wave. This can be substituted for any other Zener diode part number as long as it is a "5.1V" or "5V1" part - that's the important factor.
Secondly, yes the resistor values do need to be followed, unfortunately, as annoying as it is. Thirdly, you can probably get away with connecting pin 4 of the NE555 directly to pin 8 (positive supply), omitting resistor R3, but if things start to go wrong, this could be a cause. Finally, capacitors C2 and C3 can also probably be left out without consequence, but again, I try and do these circuits as properly as possible to prevent people having issues if they decide to reproduce it exactly.
Alternative Link:
https://drive.google.com/file/d/1Lla...usp=share_link