I once filled up my tank to top in Peshawar, and refilled it after exiting from the Islamabad interchange. It 'hardly' got 600 rupees of Petrol in it. That turned out to be about 20.8 Kilometers per liter, but it was the same reading, the same value on the consumption meter. So I really trust my trip computer pretty much.
Its average just keeps getting better all by its self. I don't know whats happening. When I bought it, it hardly gave me 18 - 19 kpl. It increased to 20, then 20.5, then 21.5 and today it was almost unbelievable.
Also, people don't get these 'readings' (even if we assume they are not accurate) in their cars generally. Getting the reading should at least imply its doing better than other cars.
Another way to verify this is to see the 'range' tab. That really cuts the accuracy down to a very tangible level as you can tell how much more kilometers your car will do with the remaining fuel.