mileage discrepency

The computer keeps a running tally of two numbers: gallons used, and miles traveled. In Europe they like to express fuel economy in terms of liters per 100 km, where low numbers are better and driving on battery alone gives you a number of zero. In the US practice of miles per gallon, driving on battery alone gives you a divide-by-zero problem and the easy way to handle the display issue is to show 99.9. However, since the car is really recording cumulative distance and cumulative fuel usage, you can get an equally accurate measure of (distance)/(fuel) for MPG or (fuel)/(distance) for l/km in either case.

 

Thanks for the explanation.

For the following, I'll assume that the computer uses the same "miles travelled" figure that is sent to the odometer, so even if it's not accurate (low tire pressure), it's the same figure we would use to calculate MPG manually.

Now I'm wondering how accurate the "gallons used" figure is. Does anyone know how the Prius tracks this? Is there some kind of measuring device, maybe along the fuel line, that's considered near-100% accurate? Or is it the same sensor that sends info to the fuel gauge (which we all know is innacurate)?

If it's an accurate measure of gas usage, then it seems to me that the display's average MPG is about as accurate as we can get, and using the old pen-and-paper method is pointless.