Reported MPG against real - can this be right?

I've recently bought a BMW E93 Convertible 325D Auto and have been looking carefully at the MPG figures. I never get anything like what the manufacturer says but I can live with that but I decided to actually calculate out the average and compare it to what the car computer says.

Over the last 3300 miles (from new), the car is reporting an average of 38.7mpg.
Looking at my fuel usage over this same time, I have used approximately 98 gallons of diesel making a true average of 33.5mpg. I found this somewhat concerning considering the sales blurb (44.1 avg) and the fact that it's a diesel so wondered if anyone had any suggestions as to why it could be so far out from what the car states.

It also begs the question about the figures reported in the Real MPG since in the past I've added my own based purely on what the car presents. I suspect most others will do the same since this is the first time I've actually bothered to record what I've spent.

Asked on 3 November 2012 by kossak

Answered by Honest John
At www.honestjohn.co.uk/realmpg/bmw/3-series-converti...7 the real average mpg is 41.9mpg. Add yours and the average will go down slightly. My 320dED records over 70mpg and has recorded as much as 84mpg. But the true calculated average over around 4,000 miles is about 65mpg. Real mpg is a mix of meter and properly recorded mpgs, so averages out.
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