People have always tried to compare mpg, and always will, no doubt, but, IT IS UNLIKELY THAT ANY ARE ABLE TO CHECK WITH ACCURACY!
I say this because there are too many variables that enter into calculations. It has already been posted about the inaccuracies of speedo trip meters, but it doesn't end there.
You don't even know how much fuel you have put in, only how much you have paid for. Pumps are notoriously inaccurate, and in order to avoid problems with weight and measures, are normally set to deliver slightly more than they show. They are spot checked, on a regular basis, and it isn't unknown for the same pump to deliver a different quantity, when checked first thing in a morning, to later, the same day.
Because a fuel expands/contracts, according to temperature, again, you wouldn't know how this affected your calculations.
Even filling tankful to tankful, can't be relied upon, because of the camber effect.
Wind speed is variable, and can have a dramatic effect on fuel consumption.
These are probably the main factors that affect calculations, but there are others.
To sum up, if you can't be sure how much fuel you have put in, or what mileage you have covered, how can you possibly check mpg, with any accuracy.
