Unfortunately, most folks are using their onboard electronics to guesstimate their fuel economy.
There are simply too many variables involved.
As is the case in my MH. The speedo reads about 3 mph faster than what I am traveling. Confirmed by GPS. So, do I adjust for this when calculating fuel mileage? No
I calculate the old fashioned way. Total miles driven divided by the amount of fuel I pump into the tank.
I average about 7.5.
The bottom line is that I am smiling anytime I'm in my MH going somewhere.