Microcontrollers are great, I wish someday learn how to use them, but my learning curve is so flat
For the most part computers and uC's are not complicated. Just a lot of very simple things you have to know all at once
I have tought kids as young as 11 to program PICs. (with a bit of help)
If you read the previous post 3v0, you will see that the problem is not how to process and display the data, but adquiring it in first place
All you need to do is have a uC count reveloutions that are directly proportional to the vehicle speed. Drive shaft, spedo cable etc. Use either a mangetic or optical pickup. Either is fine. The uC end of it is a common tach like application. Same for tach execpt your count is based on engine rotations.
A uC would very handy to light both the LED circle and the 7 segment displays.
Why do you think the electro-mechanical method using the existing speedometer is not feasible?
It could work but it is the wrong solution. How many points are you going to use. How are you going to decode the speed from the last point(s) to make contact. It is large. You ruin the original spedo.
There is another idea: if you dismantle the existing speedometer, it must have some kind of circuit to convert the pulses into voltage that drive the dial. What about using this pulses, instead of the signals entering to the instrument from the engine bay?
Yes but the original spedo in a box or sell it on Ebay. The solution I am talking about counts pulses, as I suggested there are several ways to get them.