Most cars are designed for a 400 mile range at their quoted MPG, most probably at a steady 50 or 55 mph, above this speed consumption rises rapidly. This is even more apparent with electric cars where a spell at around 70 on the motorway can see a quoted 300+ mile range drop to nearer 200.
How you drive has a marked effect on MPG, if I stay around 70mph in my Civic 1.8VTEC I can easily get from our house to Falmouth in Cornwall (about 300 miles) on about 2/3 tank- the Civic loves motorways, that is where a larger engine wins out, but local mileage I get high 30's low 40's mpg.