[Christ]

Piwo - This falls back on something I thought about, as well.

Since you're hitting all those rain drops, each one having a very small impact which occurs at a random interval, each one of those rain drop impacts is effectively slowing down your forward momentum, by driving your car into the ground with it's (albeit small) impact.

Quantify the amount of force that several hundred gallons of water might have (over a long drive), and of course, you'll see that it really does add up to some force being exerted on the vehicle. Of course, the difference is negligible, I'm sure.