To overclock our GTX 1660 Ti we used the latest version of ASUS GPU Tweak II since we are using an ASUS card.
Out of the box this STRIX card has a GPU boost reaching: 1890MHz and the memory clock speed is at reference: 1500MHz.
We were able to modify both parameters easily and achieved 1975MHz for the GPU clock and 1650MHz for the memory clock. This is a pleasing result and below you can see how these modifications change gaming performance figures. We'll be adding more graphics cards to this chart over time, which will establish which is the best at manual overclocking.