User blog comment:Dmurray1031/Luffy's newfound speed./@comment-3242083-20110503202316/@comment-3242083-20110503224251

It was never stated that Kuma is able to push things at the speed of light. We also don't know if he can regulate the speed of his pushes. Maybe it was his intention to send them flying for exactly 3 days. It would make sense, after all the Straw Hats destinations somehow fitted each of them as well.

Anyway, the One Piece world isn't real life, but it shares similarities (needing air to breath, rubber isolating lightning etc.).. So unless Oda states that light speed in one piece has not the same values, I just assume it has.

However, devil fruit versus devil fruit is another matter, as Oda stated (through Crocodile) that no one can tell how they fare when pitted against another.