I thought the speed of gravity was C plus or minus 1 percent?
I used to assume this too, but if gravity propagated at c, then the Earth would be attracted to where the Sun was eight minutes ago (and vice versa, not that this would be a big deal with the Sun).
The result would be that the two bodies would develop a couple, and crash into each other.
A full, not-too-technical explanation of the experimental proofs for the very-fast speed of gravity can be found here. Highly recommended.