My best with my HCH-I was 62.1mpg calculated with 684 miles & 11.0 gallons used. My lifetime average is 57.5 which is fine with me as I used to get 25 mpg with my former vehicle.

As for someone being able to put more fuel in the tank than is listed for the capacity, I know on the cross country reenactment that they did at cleanmpg at one point they ran a HCH-I out of fuel going into the gas station and had to push her to the pumps. The HCH-I has a capacity listed as 13.2 and it took 16 gallons. I have never been able to understand how that is possible, but knowing the people involved have no reason to doubt them. I ran mine out once and managed to pump in 13.4 gallons, but I always stop at the first click off on a slow fill, so could have forced some more in there.

 

61.7 MPG, last October.

I've broken 60 on 12 tanks, the last one in March.