h2g2 The Hitchhiker's Guide to the Galaxy: Earth Edition

I think there is a plausible reason why the warp drive scale would change from the original series to what it was in The Next Generation. The technology would have improved over the 70 odd yrs that had passed. Early automobiles probably could only go about the speed of a horse and cart, but now look at how fast some can go. Likewise it is safe to assume that the antimatter-matter reaction chamber would have been improved to be more efficient not to mention the actual engine parts so that the scale for the velocity would have to be recalibrated accordingly. In several original series episodes they supposidly went to warp 10, but that was with the engines that they had at the time. In reality they would have been going a good deal slower probably, though still faster than what their hull was designed for.