C3 Fuel Consumption Computer - How Accurate?

I have just bought a C3 HDi Deisel 90hp. The advertised fuel consumptions are 65mpg combined and 53mpg urban. The computer is reporting a consumption
of 52mpg with urban usage. The actual consumption calculated manually us around 45mpg.
My last car was a Picasso 1.6i Petrol which was also fitted with fuel consumption computer. The reported consumption was always better than the combined advertised figure of 37mpg usually around 39-40. If driven carefully it could improve to 44mpg. I must admit I never calculated the figures manually.
Why is there a shortfall on the C3 where as the Picasso was better than the advertised figures?
Is the computer system just inherently inaccurate?
Could it be down to the way diesel and petrol engines?
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So have I.

My first manual calculation was 49mpg, including a good amount of stop-start urban driving, the computer said 51mpg.
My second tankful produced 52 mpg, computer said 52mpg - spot on.
Nowhere near the published figures but the computer was reasonably accurate.
Now running at 57.5 mpg on the computer.

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I have found recently that the on-board computer on C3 is NOT accurate. I had mine serviced and the service countdown was re-set to 20,000 miles. I have since travelled 200 miles but the service facility has counted down over 1100 miles to 18,950.
Any observations, anyone.
Icedog
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Perhaps it takes into account driving style. If you have been driving like a white-van man then the computer might think you have given it 1100 miles worth, even though you have only driven 200 miles.
JPG

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I believe it is a function of time and mileage, say 20,000 miles in 2 years = 27 miles per day, if you did 100 miles in 2 weeks, you would expect it to drop more
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