I can drive 75-80 mph in my Volvo 960 and get 525 miles out of a tank of gas.
The trick is to drive at 65 or less to get over 600 miles per tank.
Oh by the way it does have a 21.3 gallon fuel tank.
When the little light goes on I can easily drive another 50 miles.
I have yet to see at the fill up less than 1.0 gallon still in the tank.