Diff ratio help

I don't know exactly what ratio diff to get for good, all-round general driving and economy for my A body Australian Charger. I have 650hp Gen 3 hemi with close ratio six speed Tremec Magnum, 18 x 40 x 315 tyres - no drag racing, driver mostly, fair bit of highway. I'm thinking something like 3.08. Thoughts?
A bit depends on the trans 1st and OD ratios. A close ratio probably has something like a 2.5 1st gear. And then the OD 6th gear is between .7 and .5. With that in mind, the 3.08 is a bit tall. With street driving in mind, my first choice would be a 3.55 gear with second choice of 3.73. This depends on the trans ratios. For good street driving, I find that the first gear ratio × the final drive ratio should come out to about 10:1. You also find about 1700RPM at 110KPH gives good mileage, a definate concern these days.
Balance the trans ratios and pick the final drive to fit into these basic guide numbers.