> I think as a starting point, I would expect that tire wear should remain roughly in proportion to road wear

IMO this would be a suspect assumption to make w/o data to back it up. You've got two dissimilar materials interacting (in very different modes). E.g. rolling a metal ball bearing on a wood surface would obviously cause the wood to degrade far more than the ball bearing, (and even a wooden ball rolling on a wood surface would wear substantially less due to the mode difference).

(If I had to guess the road has a higher wear as the surface has a tensile stress around the contact patch of the tyre, causing most of the damage, but this is just armchair engineering at this stage).