You're right, I read a few years ago that a racing bike slick tyre will not start aquaplaning until it reaches a speed of 200-250mph - not terribly likely. The grooves are thus largely for psychological purpoes.
Btw, I fitted my MTB with a set of 1.25" Fatboys yesterday. I'm quite light so i put 60psi in them and they are brilliant, fast but really smooth and great at absorbing bumps on cr*ppy country roads
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