Let's just pretend these were mountain bikes....I would guess a standard road bike weights less than 20 pounds. Even at 100 pounds they weigh less than your moped. And you are right, it is different when the weight is spread out.
Basically, it is just a ridiculous idea. ESPECIALLY with that car. Why the hell would you even consider it with a car like that. Maybe if you had a station wagon or something....MAYBE. It would still be totally ridiculous and dangerous though.
You can always get a hitch for your car and get one of these http://www.motorcycleramps.com/motorcycle-carrier-sale.htm
I have no idea if that is a good brand, it's just the first thing that popped up on google.