Mid 50's I bought very used Harley WLA 45 with junk yard 18 inch car tires-----not much fun to ride until proper 18" MC tires were mounted
80's in Germany, MZ sidecar outfit---15 inch 2 CV tire, special tube with outset stem, spoked a little outset for clearance, legal for sidecar car use----expensive way to avoid changing cheap MC drive tires. Bike worked ok and looked fine----this mod had to be recorded in title to be legal. Once tried bike solo, just down the street=no thanks for me, just too goofy handling.
My take: after 11 new maxiscooters since 1990 and over 400,000 miles with them----best to stick with OEM recommended rear tire size MC tire, proper speed rating and load rating.
Reason for strict tire wheel/tire regs in EU----the high speed there----most Americans would probably fill their pants using these speeds----Harley riders don't have to worry....their engines blow up run wide open. I lived/worked/rode for 25 years in Europe.
Having had 2 new SilverWings/2 new Burgman 400's, where car tire conversions where common.......but really never saw one being used for serious long distance riding.
One last thing, once in WI, saw Yamaha FJR1300 with car tire rear unable to make turn to gas pump, without backing up. Couldn't lean.