It's interesting how different people have different expectations on what "good" fuel consumption is. I've driven my dad's X5 35d on a 90km (about 55 miles) stretch of I-95 averaging 115km/h the whole time and still got 7,7 l/100km (30,5 US mpg). And that's with the ancient diesel engine and older 6AT. If I had the 40d I would probably do better.

I complain about the fuel economy on my X3 because it only achieves 8,2 l/100km (28,6 US mpg) on level highways and the long-term average listed below is 22,6 US mpg. For my standards this is too thirsty- my next vehicle has to do better than 5 l/100km or 47 US mpg on the highway. Salesmen look at me funny when I tell them that......but it's completely possible!

It's all in the driver technique- often I'm still one of the faster, if not the fastest vehicle on the stretch of highway. So my "hypermileing" doesn't necessarily come at the expense of speed.

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2014 Mazda 3 GT: Ø Consumption: (click for details)