THE speedo on my Toyota Yaris YR is very inaccurate. When the true speed is 100km/h it reads 91.2km/h. This means I have to either accept the speedometer reading and drive at Toyota speed; ignore the speedometer and guess my speed; or, while driving, perform calculations to convert Toyota speed to true speed. The speedo being digital exacerbates the problem. The Yaris speedo requires some distracting maths. Toyota told me the speedo falls within the legal boundaries and that they will not fix the problem. They have also said they will not endorse my changing the wheels from 14 to 15-inch.
THE law allows for an error of plus/minus 10 per cent on speedo readings and yours falls within that tolerance. While it clearly annoys you I don't think it's much of a problem and you have suggested three ways of dealing with it. If you drove at the indicated speed you wouldn't break the law, which is probably the best solution. You could guess and that would probably mean a speeding ticket or two. Or you could calculate the error and allow for it. You only have to calculate the error for three speeds, 60, 80 and 100km/h and then either remember them or tape them on your dash.