Actually, the weight of the vehicle has everything to do with its resistance to hydroplaning. A vehicle hydroplanes when the vehicle is lifted off of the pavement by water that isn't being displaced fast enough by the tread pattern of the tire. The more downward force (weight) being applied by the tire, the faster the vehicle needs to be moving to develop enough water pressure to overcome the weight of the vehicle, given identical tires. A larger contact patch on the tire means less weight per square inch of contact patch, so the water doesn't need to provide as much force per square inch of contact patch to overcome the weight of the vehicle, which in turn means it can lift the vehicle at a lower speed. A heavier vehicle means a larger contact patch (and as a result, a wider tire) can be used, since more weight is being applied per square inch. Instead of using a narrower tire to make for a smaller contact patch, treads can be designed to channel water away appropriately, compensating for the larger contact patch.
Now, the difference in wet performance between a 16" tire and an 18" tire, given the same width, would be determined differently, having more to do with the angle of the curve of the tire vs the angle of the road. The difference between these two tires would be minimal. Conclusion: Given identical vehicles, with a such a small difference in tire diameter (two inches), resistance to hydroplaning is more of a function of tire WIDTH than diameter. You don't need an engineering degree to use deductive reasoning. =P -Ivan
