is the speedo on the outside of your tyre, no, so therefore it makes not the slightest difference,.
I think it would only be you who are liable.
Heres a calculator to work out you percentage change though!
if i fit after market low profile rims to my car, does it affect my speedo, if so could you use this to overturn a speeding ticket, with the case you did not know this? i thought wheels was wheels!