I track my mileage and apply some conversion to tire size etc. I built a Filemaker app for truck maintenance and it does the math for me based on miles driven (+ tire conversion) divided by gallons used (or added to the tank).
Globally over the last 12 months, calculating every tank over a range of 10k miles, 10.92MPG average. I have seen single tanks as high as 16 and as low as 9.5MPG. My average takes into account a lot of city driving