DirtyPiere,
You are quite correct - this applies once the wheels have spun up, untill then you must take rotational inertia into account - it's quite simple, the greater the mass of each wheel, the more it will resist 'spinning-up', and of course, untill it has reached full rotational speed the tyre is by definition hydroplaning or aquaplaning (even on a dry rwy - it's called reverted rubber hydroplaning) - instead of getting caught up in the formulae, consider the basic physics. The extreme example of this effect would be landing with the park brake on - does that clear things up?