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I'm not entirely sure if you have a problem, but your computer doesn't (I believe) have an instant fuel readout - what you are looking at is the average MPG. This will change more readily at the earlier stages after being re-set, and then changes will start to slow down in frequency as time/distance elapses.
What I would suggest is to brim the tank - re-set the computer & trip counter, then drive as you would normally until your next fuel stop - note the miles you have travelled, and the litres needed to brim the tank - divide this sum by 4.54 to convert litres to gallons, then divide the miles covered by the gallons used - this will give an average MPG figure. Which can then be compared with the computer to assess its accuracy.
Don't use the brochure figures as a benchmark - they are based on laboratory conditions, but check simple things like your tyres are inflated correctly, and that you aren't carrying excess weight in the car or a roof rack. Also, take into account congestion and delays adding to the time spent idling in traffic.
Once you have your own definitive figures calculated, I'm sure you will be able to assess your car's health.
Hope that helps,
This job should never be complete - the day I think I've done it all, is the day I resign.
Currently in VelSatisfied's garage:
2005 FIAT Ducato 2.8JTD LWB Gran Volume, 2000 Kawasaki ZZR 1100 D7 'fullpower' in black + full GIVI & KAPPA luggage, 2007 MB R-Class 320 CDi Sport LWB uprated by Brabus to 300Bhp don't know if there's a 155mph limiter - will be fun finding out! 2004 Mercedes Vaneo 1.7 CDi Ambiente 7-seater