Hi
I've been away for a while but just logged on in an attempt to help someone with a problem. While I'm here I thought I'd mention that last summer I managed 779 miles before the miles to go indicator went from "30" to "---". Added together gives 809 miles to a dry tank and the indicated average mpg for the tankful was showing 61.8 mpg. Just saying. 
Mick
I think your economy was far better than the trip computer showed.
If there was enough fuel in the tank when you had completed 779 miles to do a further 30 miles and your car was averaging 62 mpg - then there must have been 30/62 = 0.48 gallons left in the tank.
The most fuel I managed to brim my FD i30 with was 55 litres = 12.1 gallons.
Hence, assuming your tank was brimmed, you would have used 12.1-0.48 = 11.62 gallons during the 779 mile trip
Therefore the economy was 779/11.62 = 67 mpg (4.2 l/100km)
Congratulations......
