22.5
The Earthroamer's weight would be too much for the Toyos mentioned as well, if used with single rear wheels.
I think the decision to go with the 22.5 inch tires is to provide tons (literally) of extra capacity. That way there is no "lawsuit waiting to happen" when someone takes the starting offensive line of the Denver Broncos for a ride in an Earthroamer.
The real solution is, like Charlie Aarons keeps telling us, to use a tall, narrow 20 inch tire (narrow so the front wheels will turn without interference). The differentials would have to be re-geared to compensate for the much taller tires, and you might have to upgrade the axles to deal with the extra rotating mass.
The increase in cost would be small, compared to the Earthroamer's $200,000+ price tag.
Chip Haven