Yes, people apparently have been known to fit the 165/65r15 KR20s to Smart cars. Smarts have used an almost infinite variety of tire sizes (and wheel sizes) in various models since they first came out. I would imagine 4 KR20s would be OK for tire loading as the Smart car is not much more than 50% heavier than a RT LTD. It would need more pressure of course. I doubt dealers would fit them, but Smarts are often modified by owners in an attempt (usually in vain) to get them to not under steer or over steer depending upon the model. Having driven a couple, I can safely say the Spyder is probably the much better handling of the two. I thought the Smart car was just plain dangerous especially in the wet, like a Reliant Robin with 4 wheels. They tried to swap ends at the slightest provocation. I think the model I read that had the KR20s fitted to it was a Passion Cabriolet and the owners were in Malaysia if I recall, as whatever their OEM tire was, they couldn't get any. In the way we look at car tire alternatives to provide better solutions, apparently some Smart car owners have been experimenting with the other way round. From what I understood, the logic is that it is felt the Smart car is too light to work well with conventional car tires as it can't get them up to a decent operating temperature and their sidewalls are too stiff (they do ride like a jackhammer to your spine). This suggests they must be able to get the KR20s from somewhere. I don't suppose those willing to experiment in such things care much about the legality of it, the same way some Gold Wing owners fit car tires to the rear of their bikes. Perhaps running them at a higher pressure than 18 psi, they might be a reasonable alternative for some Smart car models, I don't know. I just thought it was kind of funny that something we are trying to get away from as of questionable quality is something that other groups were looking to as a "performance" alternative because they are expensive (and hence "better") and they couldn't get the OEM tires because of supply chain issues.