It works out you theoretical range based on the average economy of your most recent journey.
If you have been trundling down a duel carriage way in the lorries slipstream at 50mph for the last 20 miles, you could well have been averaging 45mpg. If you then pull in and fill your tank (55 litres), the computer will assume that if you carry on averaging 45mpg, you could travel for 545 miles on that fuel. If you spank it away from the petrol station and average say 20mpg over the next 20 miles, the computer will recalculate, at reduce your range to empty down to 240miles withing that time.
It can only do it's sums based on the information that it has received from how you have been driving and assume that you are going to continue that way.
