I believe there has been an improvement in the Derby on the TG sheets from the 5-6 range in the early 80s to this year's peak.

I don't know the exact formula, but that's probably in the range of 10-12 lengths at 10 furlongs (Jerry can tell us exactly).

There was some discussion about pre 1982 horses like Seattle Slew, Bid etc... on another thread where faster figures were mentioned, but those were "RAG" figures.
There has been a slow change in the relationship between Rags and TG over time. So not only are those fast figures not on the same scale at TG, the scale itself has been changing. TG figures used to be slower than Rags. Now they are faster.

I think it is best if we refrain from comparing figures from different figure makers (pre TG) when we discuss if horses are getting faster. Otherwise the conversation will deteriorate into a mush of misunderstanding. :-)