the 16" to 20" motorcycle to bike tire thing works because the rims are 405 and 406 milimeters. I forget which is which. American Bikes measure the inflated size, Motorcycles measure the rim size, and the french had there own system that i fail to understand. Lucky my Peugeo 103 was sold as a motorbike, so it uses motorcycle standards and 17 inch rims. your Mobylette i think was sold as a bike with Motor, and the rims could be based on bike sizes. Crazy French.
MopedArmy.com is the first place to ask. Sheldenbrown's website explains the sizing some