Is this an issue at tracks/races outside of the US? If not, are there any concerns that going to a spec tire model in the US would kill foreign participation at our large races? A small portion of our entries, yes, but something I am curious about.

Blue/Double Blue or Blue/Blue would probably be the best combinations that would work for the whole spectrum, from 17.5 to mod. Alternately, what if you made it so there were two spec options for rears and two for fronts? For example, you made both Green and Blue spec rears, and then both Blue and Double Blue spec fronts. Racers could run any combination of those tires - still allowing some grade of tire tuning, but keeping the options minimal to keep cost down. Only issue I can see is that large races would have to figure out how they would approach ordering tires, or even discuss offering a limited selection (eg, Birds only uses the Blue/Dbl Blue option.)

I also have some concerns over the spec tire question for mod at large races, but I think that debate is a long way out given current mod participation within the US.