This means that the GTX 1660 needs to add some decent margins over the GTX 1060 6GB. The GTX 1660 Ti and GTX 1660 are roughly similar in terms of clock speeds - in fact the latter offers higher base and boost frequencies. The GTX 1660 Ti performed excellently and was well ahead of the RX 590, leaving the GTX 1660 snapping at the AMD card's heels. There was between 10-15% between the GTX 1660 and GTX 1060 6GB too - again Nvidia was right on the money here. Nvidia's statement that performance is around 15% better than the GTX 1060 6GB is maybe even underselling the GTX 1660's performance.

Source: Forbes March 14, 2019 12:56 UTC