I’ve only got 315/70/17 KO2’s ( that measure about 34 1/2”), but at 60 it holds 8th pretty well. With a Tazer to tweak in the speedometer to agree with gps, the tach shows 1500rpm. With enough headwind and mild grade it will sometimes drop to 7th. I can’t imagine it needing 7th at 80mph. Hopefully other diesel owners will chime in, having sustained that speed on an interstate and can offer first-hand 80mph data.That's the math, but does it actually do that or drop into 7th?