I have a spreadsheet with all my MPG and maintenance info, and on average the actual values are running about 5.2% below the computed values. In individual tanks, I've had differences as high as 10% between the two, and as low as 0.7%. We're not talking a ton of tanks here, so I wouldn't treat this as normal expectations for your situation.

More imporantly, compared to if I had continued driving my old SUV, I'm averaging a savings of about 9.5 gallons per tank (assuming exact same driving needs and that the SUV could hold enough gas to make the same mileage trip as the HyHi).