Wiki says US pumps are certified to .3%
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In the United States, the National Institute of Standards and Technology (NIST) specifies the accuracy of the measurements in Handbook 44. Table 3.30 specifies the accuracy at 0.3% meaning that a 10-US-gallon (37.9 L; 8.3 imp gal) purchase could vary between 9.97 US gal (37.7 L; 8.3 imp gal) and 10.03 US gal (38.0 L; 8.4 imp gal) as to the actual amounts at the delivery temperature of the gasoline.
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Why would you be so surprised to find that the computer read out in the Insight would be wrong? My computer says 58 mpg when the tank fill says 55.5. It is just a computation based on samples of engine load via the MAP sensor. Not nearly as sophisticated as an MPGuino which counts injection on time. But then that has to be calibrated by comparing to... actual tank fills.
