For your speedometer, a simple option to consider would be putting a few marks with the corrected speed of several common speeds that you travel option and covering them with something like a screen protector used for a blackberry/iphone/kindle, etc.
For me, I would probably care mostly about 35 and 55 mph marks, since most of the roads around here are those speeds. 45 could be interpreted as 1/2 way in between.
Not that it matters much- i rode an old harley for years here that had no speedometer and just stayed with traffic- never had an issue with the law....
1980 Vespa P200E