I suspect the easiest thing to do is pop the cover off, pull the needle off, find somewhere long, straight and empty, and refit the dial when doing a constant 20 or something..(I believe they pull off/push on?)
It'll be on my list of things to do, but suspect I won't get round to it this weekend.
Could, of-course, fit much lower profile tyres to compensate

Hmmm - a thought - if the speedo is overreading like this, is the odometer likely to be doing same?