Comparing average mpg is meaningless really unless the two people drive for a similar amount of time and at similar speeds. Lots of small trips will kill your average mpg, particularly with diesels which need time to become more efficient. Someone told me that the petrol Q5 gives the same if not better economy than the diesel if used for lots of small trips
Re. the official urban/extra urban figures: they are useful only as a comparison from one model to another tested with the same procedure. The actual driving sequence under test conditions bears little resemblance to real driving. It's on wikipedia if you're interested
Also the X3's figures are for non-run flat low rolling resistance tyres, not run flats as most people will have
All of that said, the difference between the official mpg and the actual mpg seems much greater in the X3 than it was for example in my last car (Golf GTI). I distinctly remember getting 38mpg on the motorway at about 75-80mph in the GTI and the official mpg is LESS => 35mpg. Compare that to the X3 and it would be like getting 54mpg at 75mph. Absolutely no chance of that unless you're going downhill!
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