Re: GT Diesel - Average Fuel Consumption
The only way to really know what your consumption is, is to get your tank fairly empty, fill to the brim and zero your trip meter. Then drive till nearly empty again, refill to the brim and check the trip meter. You then know you have done exactly x miles on y amount of fuel. Repeat 2 or 3 times to get an even more meaningful average, your journeys may vary slightly from week to week.
The 'consumption meter' in the car is only really meaningful as a 'spot guide' as to how much fuel you are using at any particular moment, for example comparing consumption going uphill to on the flat, driving at different speeds or at a certain speed in different gears. On those occasions, it will give you meaningful information.
If you want to see how accurate your car's computer is, try the method in the 1st paragraph and compare it to what the computer thinks - you may get a surprise!