My car’s mpg reading is overly optimistic (it shows better mpg than doing the math myself).
My NC700X is pessimistic. It reports worse numbers. I actually get about 2 mpg better doing the math myself.
The best I've ever done was a ride through the Sierras where I calculated the use at 84.6 mpg.
That's all high altitude (peaks near 9K feet), low ( 35-55 ) speed riding.
Down in the valleys on the highways running 65-70 mph I'm usually in the mid 60s.