I just did a 2600 mile road trip, lots of blizzard stuff and 4 wheel high driving and I zeroed all the gauges to do a real world test. My on board said that I had averaged 18.2 mpg for the whole trip and the math came out so close that the difference was in 10ths. I think the on board stuff is pretty accurate!

 

my onboard calc is 16.7 in the city with hills and all. my actual fill-up calc is 18.5.