How would account for differences in scale? Not so much size (although certainly Diminuitive/Hulking Size can be a basis), but scales between toughness and speed and such? Would a tank have Mythic Toughness with a Catch of "Anti-Tank Weapons", for example? Do all cars implicitly have Superhuman Speed or the like so they can outrun a WCV but have trouble with a BCV?
Excellent points. But the OP's exercise is to avoid having to fully stat out vehicle and mounts.
In the FATE Fractal, yes, we would build the vehicles using the same powers as characters, and when the scale gets beyond what character-level powers can/should do, we "zoom out" and set a new standard for Mediocre (+0) when we build things on the higher scale. So the DFRPG rules would probably work for anything up to the size of an APC, bus, tank or small yacht. But once you get into aircraft carriers, cruise liners, cargo ships, space stations, mobile cities, flying islands, massive space ships, "Hulking Size" doesn't adequately cover it, so we zoom out and figure how to multiple everything in relation to the next lower point in the Fractal.