[dcb]

the second example, coasting back with the engine off, highlights a key point though. It is not instantaneous mpg that is important, but average. If you use a gallon of gas to go 5 miles up river and float back, the trip mpg with the whizwheel will be something between 5mpg and ??? (because it did not accurately detect the return trip and you spent time in different currents, maybe you were anchored for a while and it kept spinning), but with a gps the Actual mpg will be 10mpg. The drifting downstream at 999999 is terribly important for your trip averages.