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Thank you for your replies.
Fuel consumed during idling will make a slight difference, which I had not considered--good point--although I suspect it is not 10% of total consumption. Tire sizes affect the speedo reading, and it is common for speedometers and odometers to overread, even with stock standard wheels and tires. GPS speed is very accurate, however, typically within a fraction of a kilometre (or mile) per hour. Doppler shift is used to correct for the unit approaching or driving away from each satellite in use at the time. Speed is calculated several times per second, although some devices may only display an average of the last three or four seconds. Unless one constantly drives only uphill, or only downhill (which we cannot do for long) the minute differences which might result from climbing or descending a hilly road are almost unmeasurable, and uphills and downhills should cancel out. The GPS satellites are orbiting about 12,000 miles, or about 63.3 million feet, above the earth, so even the instant speed difference due to vertical road changes is infinitesimal.
Have you used Fuelly, or a notebook, or some other method to compare actual consumption to the displayed averages? I'm still left with a rather large and one-sided difference between real and displayed mileage.
 
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