Going to get some arguments here, but what the heck

For the last 12 month I have been resetting my trip distance and average MPG on the computer every time I fill up (LPG). Each time I record the trip miles and the avg MPG on the receipt and calculate it later.
The ave mpg is dead on accurate every time as it is displaying the avg mpg for
that tank of fuel and the way
I drove using it.
If you do not reset the avg mpg each time you fill up you are getting the average for every tank full and the way you have driven since it was last reset. IMO by what I have found, this can cause manual calcs to be slightly different from the computed
I have found a couple of things doing it this way
When I set off from just filling up and the avg mpg being zero'd I drive a little better as initially you will be seeing 9-15 avg mpg

If you come to a jam or lights fairly quickly you see it going down at tickover, ouch, but it slows down, going down, if you put it in neutral

I find myself constantly trying to get the new avg mpg up as high as poss all the time, which can only be good

By the way get 22-23 around town, stop start, short journeys. On the Motorway 35. Cant complain, and wont, it's my mig

And yes I know there is some petrol usage in there and I dont record it as it is minimal and the results have made me trust the computed results.
Give it a try Mr Bear