The R129 300SL's came originally with 225/55-16 tires.
Twisting the stem of the speedometer needle changed the displayed speed on the older models with the Speedo cables. I doubt that anyone has messed with the speedometer needle on your car (you can check this by taking out the dash cluster; the cluster-parts have to be sealed with a sticker).
Personally I don't think there is a way to calibrate the speedometer...but I am not a mechanic!
with a minor 600+k