I made a print that I simply stuck on outside of the glass of the stack. I made it small enough so it would fit inside the mile/h scale. I got it in powerpoint format and it prints out to scale. For the moment I printed it on regular paper and stuck it on with tape. I'm still looking to print it on clear sticker material, because now it doesn't look as neat, plus the paper doesn't let through the backlight from the stack, so at night it is difficult to read the speed.
But actually I don't use the speedometer that much. I tend to use the in dash GPS (JVC KD-NX5000) a lot more to keep track my speed in km/h. It displays the speed in big numbers.
In case you can settle for the 'stick-on' solution, I have the ppt somewhere, I can dig it up if you like.