It turns out that converting a car to three wheels means it is legally a motorcycle. So how to do this with a Gen 2 Prius? Weld two rims together. Reverse the rear suspension parts to support the central dual tires. Sure. No walk in the park. Yet also not so difficult. Why do it? You can aero mod the old rear wheel openings. Mileage goes up. You might use the wheel well volumes for storage. Insurance goes way down. Possibly you can avoid smog requirements. Finally, it will be a real conversation magnet. P.S.: 2 tires on the now single rim is legally a third wheel and NOT 2 wheels. Per modders who passed DMV.
When I lived in Germany I learned, among other things, 2 facts. 1. Many Germans are very thorough. "Ordnung und System". 2. Vehicles are *required* to keep a maintenance log. Things are strict there. So. Check this out.
Forget the (WHY?) It's the details of the (HOW?) I want to know. If the rear tires are now a single unit in the middle- how would you mount it on the car, check or adjust air pressure, or anything else? How would you put in a central "wheel well"? Posted via the PriusChat mobile app.
Could you just hang a new rear tire off of of the back, like a motorcycle tire? Similar to something like this: https://bringatrailer.com/2017/02/27/early-goldwing-powered-trike-1984-tri-magnum/
Just call ADAC in Germany and ask them. https://en.m.wikipedia.org/wiki/ADAC Information: 08005-10-1112 (Monday-Saturday 0800-2000)