I'm wondering how I go about creating a Dial Gauge Tach and Speedometer for the overlay, as opposed to the standing "digital" style.

You mean an image that changes according to speed/rpm for example? It is possible to do this with dynamic images although a bit tricky and could be hard on performance. In general the steps for example for a RPM gauge would be:
- Have a list of images that correspond to various RPM values, in appropriate steps for example 200-400-600-800-1000-1200-etc.
- Name them all according to their rpm: 200.png, 400.png, 600.png, etc.
- Use a script to round the actual RPM to a multiple of 200 (divide by 200, round down, multiply by 200).
- Let the script return the rounded RPM.
- Apply the script to a dynamic image path.

There are examples for all of these steps in these forums somewhere I think.

Ah ok, thank you for the clarification, I dug through yesterday trying to find the info on it. I'll look again, mainly for the script step, the other steps I think I've got.

It is also possible to make a linear movement of an object (like a cursor). You do a script wich transform RPM or SPEED into number of pixel and use it to override left position of your object.
video

Refresh frequency of RPM and KMH are not the same. With RPM we get dynamic information and speed information come with some "delay". Low frequency.
Is it possible to get it like RPM ?