Just made a 700 mile trip to dallas from Little Rock. Topped off with 13.2 gallons of 93 when the LOW FUEL light came on. Made it 340 miles before it came on again
This was 99% highway too, driving about 2800-2900 rpms the first 150 miles and then 2300-2400 rpms the rest of the way until the light came back on.
340/13.2 = 25.75 mpg, all highway with 4.10s.
Now, its not an exact # because theres no way of telling exactly how much fuel I used. I just went from LOW FUEL light to the next LOW FUEL light, and there was still a good 20-30 miles left if I wanted to risk it.