To the 18-inch faction on the B4!

Hello everyone The Odyssey takes its course. Had actually had to register before next week 18 inches on my ́93 ABT B4 and that with 225/40 tyres. Was earlier at the speedometer check on a test bench for the required paper for the Tüv. The speedometer shows, what is understandable because of the larger roll circumference, too little. At e.g. real 120 driven km/h the speedometer shows only 114. Unfortunately they did not get it to me to put the speedometer on or after. (Was a speedometer?) Opel car house with the only test bench display here far and wide, had no other possibility) Now I am sitting quite nicely in the Bredullie. Do you know perhaps where I can have the speed indicator adjusted in the area Hannover in my speedometer? On the net I find somehow absolutely nothing. To me all the companies dealing with this kind of thing have emigrated because of the new legislation in DE all abroad. Although I do not want to adjust my mileage, what is forbidden I was set to problems with the size of the rims and the tyres/cross-cuts but such a cheese. Would be very grateful for any help!