For hwy driving I would rely on a GPS of some description, The Aust Gov gives car companies a 10% error in speedo readings
The Victorian Police allow you only 5% if that, do the math lol
I run 285s and find the speedo near enough to correct,
when I fit my 315s its out same as when I had 265s on it
For hwy driving @ 110kph Ill set the CC to 105kph Im in no rush
and my ETA in marginal over 2.5 hrs