I actually feel like everything is going well so far, though I can't say that the way that we are implementing the changes is the cleanest. I realized that I had to make some XML modifications in order to lay out the menu option for speedometer types (which I put in the simulation menu for the time being).
I added the gauge loading. It now loads the proper gauges and I think the angles should work once we complete conversion to KPH in grboard.cpp (which my partner for this project is responsible for).
The files that required substantial modification were grboard.cpp (for loading textures and ultimately outputting speed to screen), simuconfig.cpp (for adding the menu option), the XML files for each car, and simuconfigmenu.xml .
We wound up using if statements within grboard.cpp to decide which speedometer to load and speed type to calculate - not the most elegant solution, but it does the trick for now. I have basic mph-style speedometers built for the default and three of the cars and intend to do the rest over the next two weeks.
Just let me know what you need from us in order to implement this in the official build!