Mpg calculations?
Answers:
They do not drive cars around and work out an average. They have the engine on a "running rig" where they "simulate" different conditions e.g. Urban, Extra Urban, Combined thats why they are usually not very acurate. (this is in Europe, I dont know about other places)
All new car models and their engines are tested over long periods of time to give mpg .
I believe that they take multiple cars of the same make and model.then just drive them around and take the average of the test results from different areas.
I haven't researched this but I would guess that they do actually drive the vehicle, using a specialist driver who drives as economically as possible therefore producing the best figures. Whether this is done in a real world situation or using some sort of 'test area', I don't know.
Apparently there are EPA testing rules for how this is measured. See http://www.epa.gov/fueleconomy/data.htm.
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