One thing that no one seems to addressing here is that the water is not a solid weight attached to the tires. It is a liquid floating, so to speak, inside the tire. When the tire turns it will slosh around but not completely rotate with the tire as if it were attached (At lower, off-road speeds). For example, if you fill a small tire halfway with water and spin it, the water won't rotate with the tire, it will stay at the bottom (untill the speed gets high enough for the surface tension to overcome the weight of the water). This should negate some, not all, of the effects that are associated with higher rotational weight.