When I got my current tires, which are 205/60/16s at 25.7" tall, I looked for a 195 option, which doesn't exist in 16" wheels without dropping below my stock tires' 25.5" height. Now, there is actually a 195/70/15 option that is 25.7" tall....I could combine that with the 11 pound 15" wheels from the 1990s mazda millenia. Unfortunately, the only tire types available in these sizes are van tires, which are designed to carry a crap ton of weight. This may actually be not so bad, since I would assume a tire with better load carrying capacity can have lower RR. Plus these van tires are rated with max psi between 65 and 75!
My only concern would be the strength of the rim. Would it stand up to those pressures, or would I have to grab van rims as well? This is probably the closest we can come to trailer tires legally.
Speaking of trailer tires, how low RR would they be? They seem to sacrifice grip and wear, so they must have low RR, plus they can carry a ludicrous amount of weight.