Hi Alan, I have a i30 1.6 Crdi Style estate 6speed, The best computer mls to empty never goes above 489mls yet I get 550-600mls easily and when on a long run (motorway) it may say 329 mls to empty then when I turn the car on after a stop it goes up say 389 or so, I have let the light come on and gone down to 30mls until empty on the trip, I had 639mls when I filled up, I got 51litres in brimming to top.
I think it seams quite accurate. Today I topped up with 3/4 tank full had done 210mls and only put in 15litres!
Whatever the needle on the gauge says I tend go by I know of some who run the car by the mls to empty but have run out ! At the end of the day I don't like running my car low to light on as the crap in the bottom of the tank.
Just my 2 pennies worth.
Cheers Sean.