I am not sure if you would count this but on a recent trip I had - taking I-5 from L.A to San Jose. I averaged about 80/85 mph and was able to achieve 85-90 miles per quarter tank - I was using 91 octane and I-5 is mostly flat for the most part - hopefully that puts things in perspective?
Also note that the entire tank is 13.2 gallons - I am not sure if the reserve tank is 3 gallons or 2, but from what I have experienced it must be about 3 gallons, which means that the tank size before the 'E' light turns on would be roughly 10.2 gallons.