Thanks, all good ideas.I would think that between the different sizes of the face and differences in increment spacing of different speedos it would be impossible for someone to make a one size fit all overlay.
With that said I would look for plastic film like a phone or tablet screen protector and maybe using a permanent marker marking out important mph lines such as 30, 55, 70 and 100 something.
Or get accustomed to kph conversion because as you know when us baby boomers were in elementary school we had to learn metric because pretty soon after that the US was going metric, lol.
Another thought would be a GPS or phone that has speed output.