Millenium, that claim of 700 to 800 is to empty and is an extrapolation of distanced travelled, fuel pumped in and remaining fuel.
thus, say 600kms on highway then fill up say 55litres, so actually still had 71- 55 = 16litres.
now 55/600*100 is 9.17L/100k. Now assuming the remaining16 litres was used at the same rate will get another 174kms, if my math is correct thus from full to empty is 774kms.