Your posting name suits you well, doesn't it? Under Blair's new rules that he's going to try and railroad in 31mph will matter. He would like to see zero tolerance.
If you know the size of the standard tyres, and the size of the new tyres, you should be able to figure out who faster or slower (as a percent) that new tyres would be compared to the standard tyres. Then, you should be able to figure out how many percent to add or subtract to speed to make the speedometer accurate.
In the states, sometimes police put up radar guns that show drivers how fast they are going. If you have those in England (or whereever you are), you can get the new tyres and use this to calibrate your speedometer.
Jeff
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