A few years ago I kept a log of my H3's mileage between each tankful ...for about 6 or 7 tankfuls. Filling up at the same pump each time. Overall mpg came out to be 17.4 mpg, a blend of city/hwy, commuting, shopping, etc ...my wife driving sometimes too. But one or two fill-ups in my log which were mostly highway miles landed in the 20-21 range like you observed. Not too shabby considering it's an I-5, the weight and brick-like aerodynamics of the H3.