Taken from the computer. Their pretty accurate on my 182.
When you hit the reset button, it takes a few hundred meters to pick up a reading. When it does its basically what your doing then and there, but after time it starts to get the average since its been reset. If you see what i mean. I was wondering why the fuel was going down so quike, checked the mpg and it said 31.5 !! Bit low for a 1.2 lol. Its soooooo slow.