The way I figure it is, I check my mileage religiously on every single fill-up, and I pretty consistently average 23 mpg, with little variation (although once a year it starts to go down, which tells me to clean my K&N). So I figure that the tank is what, 14 gallons, right? So I know that conservatively, I'll be safe to 280 (which works out to 20 mpg), and if I wanted to stretch it, I could push up toward 300.
But I usually get scared around 230-240 and fill 'er up. A few summers ago, I bombed down to N.C. in around 10 hours and was getting 300+ per tank, and I kept it right around 80-85 mph the whole way down.
