I see a few.... "I get about....."
Keep track of all of the fuel you purchase and read the total miles off your speedometer (assuming that your speadometer is acurately calibrated with the tires that are on your vehicle.)
I choose to use the total amount of fuel consumed and the total miles driven since day one.
Reason... from home to the gulf coast is downhill... for example... I might get 2/10 of a MPG better in that direction... returning home is uphill... and I will lose anything on overall average that I gained on the way down,
Your trucks computer should be very accurate,,,, check it with your own calculations... if they are different ask yourself... did I have my odometer recalabrated after I put the 19" wheels on the truck"...
Something not many people think about.... New tires go further in one complete revolution that badly worn tires... new tires are bigger in diameter... not much maybe but over 40,000 miles it would add up.
Even the type of fuel from station to station would add up to gains or losses over time.
My truck and truck camper have been set up close to the same weight wise since 2003... GMC one ton Sierra SLT Duramax/Allison still set up like it came off the factory floor.
Running 80 PSI tires (important), and the majority of the time at 58 mph (almost never above 63 mph) with the TC loaded more than 99% of the trucks life time:
... hand calculated from day one we had 15.1 mpg until March of 2015... this years exstended trip in the mountains of New Mexico and the slightly higher MPH heading home on I-40 dropped us overall to almost a flat 15 mpg. Confirmed by the trucks computer.
What ever MPG you get.... make them count... go somewhere fun.
Summer is half over...
Chet
2003 Lance 1161,/slideout/AGM batteries/255W Solar/propane generator/Sat dish/2 Fantastic Fans/AC/winter pkg
AirFoil, Trimetric, LED lights, Platcat vent heat
2003GMC K3500 LT/Crewcab/duramax diesel/allison/dually/4x4/OnStar/front reciever mounted spare