How accurate is the mpg readout on that display screen on the Prius. How is it measured. Sorry, I don't know the technical jargon for it. Thanks.
Folks who do manual miles/fuel used calculations report the readout to be ~3-5% high. This may be the result of most country's auto regulatory bodies mandating that speedos read 5-10% high to try to keep real-time speeds down. See ENERGY MONITOR on page 3-3, here. Read the rest of the PDF at your peril! No problem. In time, the Prius-specific lingo will come naturally.
Complications: - It's perfectly possible to make the odometer (which the MPG meter uses) completely accurate and still have the speedometer read high. It's all just data in computers. - Gas stations have an economic incentive to overstate the amount of fuel delivered, which will make manual MPG calculations too optimistic. - "Topping off" defeats the vapor recovery system and allows some fuel to evaporate, which reduces MPGs measured by every method. Unless someone has made multiple measurements over many gallons which they themselves measured (that is, not trusting gas station pumps to be perfectly accurate), and over well-calibrated distances, manual versus MPG meter comparisons will be uncertain.
