I always just calculate off the trip meter, reset at refueling. Like I said, it's all relative. From full, I was lucky to hit 200 mi on the trip meter to low light, which usually seems to be 21-22 gal to full. Figuring I'm about 9-10% off, that still puts me right around 10 mpg... however, I'm a bit on the heavy side (had roof top tent on, front steel bumper with winch, rear steel bumper with 5th wheel swing out), and always keep ECT on and have a heavy foot, along with the mitigating factors I mentioned previously (cracked coils, cracked exhaust manifold that was getting worse)