Tach hours reference speed?

I thought of this because my tach shows I’m 80 hours past TBO but if you apply the 1:1.05 differential I’m still under TBO!!

I asked Grok. The answer was an airplane flying at 2400 rpm with a C-85 engine and a C-75 tach could record 1.05 hours for each engine hour.

This one has got me. My airplane had a C-75 when I bought it. A couple years later I had it upgraded to a C-85. Nothing was done to the tach. 

The hours displayed on the tach are calculated by the number of turns on the engine referenced to its typical rpm at cruise. Redline on a 75 is 2275 and redline on an 85 is 2575. So I am assuming that the hours on my tach are higher than they should be, since I’ve been cruising at 2400 rpm, 125 rpm above C-75 redline?

is there a different tach for each small continental?

Am I even on the right track here?