On a 255 mile motorway run starting with a full tank, getting better than 40 average mpg, when we near our destination the instruments show a near empty tank and show 40 miles to empty tank. The tank capacity is 10 gallons and we should get better than 400 miles range. Sure enough on refuel I can only get 33 litres in. So I'm carrying around 2.5 gallons I daren't use and being cheated out of a quarter of the car's nominal range. Is there anything we can do about it?