Diff ratio help

yeah because nothing screams economy like a 650hp G3
I only need sixth gear for long haul freeway driving to save fuel, and drone of too high revs for hours on end - I.e., a trip from Canberra to Brisbane return, which I do every couple of months in one day, is about 1200 klms (720 miles) and 13 hours each way. The old car I had was LSA, about 480hp, 20 inch tyres, tr6060 six speed manual and had 3.70 diff, which was factory. Good economy long haul, good power throughout rev range. I thought with more power I'd go lower diff number, but advice here is go higher? People now have me thinking 3.9 or 4.11. Engine EFI is managed by Dominator X Max, if that helps. At shows, all I wantbto do is park it and drink beer, not cruise and burn tyres. I'll save that for elsewhere. But for me, this car will be a daily driver. I live 35 miles out of nearest city on a major highway, so not much city driving to worry about, and even then I don't care about fuel. It's only really the highway driving and long trips that I care about as that is 75% of my driving time.