I find that VERY hard to believe. You sure you're not reporting Kilometers per Gallon ;<).
Also, while it is possible that for a period of time at high speed, going downhill for example, that the car will run on batteries even at high speed (I've seen this at speeds over 70 MPH), and that this will generate PEAK gas mileage figures that are very high, the average of a full driving cycle is what we're interested in here. Real world, day to day stuff.
Thanks,
Neuromancer