i don't know about the VEs, but for my VP i can calibrate the computer to change values such as fuel flow etc. i am going to calibrate it tomorrow to show you guys that my old worn out v8 only uses 2l/100km. if i post photos will you believe me?
the point is that computers are inaccurate. even new ones. what you need to do is fill up your tank full, then reset your trip computer to 0kms. drive until you need to fill up again. then fill up full and write down how many litres you got into your tank. now use the following formula to calculate your economy:
(litres used) / (kilometres travelled) * 100.
never ever believe what your computer tells you because it's most likely telling you lies