There was a thread about this some time ago.
The law states that speedometers can read a percentage plus a fixed value over (e.g. 10% + 6mph) but it is illegal to read under by any amount, so manufacturers always calibrate speedos "fast" to avoid breaking the law.
Tyre wear has very little affect on speedo reading. I did calculate it once and it was less than 1% from new to knackered, and it makes the speedo read faster for a given speed as the tyres wear, so for the manufacturers calibrating with unworn tyres is failsafe.
Note that the Vee and Wee use different methods to drive the speedo as mentioned before, so changing the sprockets or rear wheel size on a Vee will put the speedo way out, and so you need a speedo-healer, whereas it has no effect on a Wee. However if you change the
front wheel size on a Wee, it will put the speedo out.
If I wasn't so lazy, I would link to the old thread, but I will leave that as an execise for the reader.