Why does the fuel economy differ by 9mpg between two identical Ford Focus?

My wife and I have identical Ford Focus 1.8 Zetec diesels, one four and one two years old, and we often swap. The tyres are the same make and same pressure, and both are fully serviced. After extensive testing over a year, I achieve 52mpg in one and 43mpg in the other. The route I use is always the same, a commuting run of around 12,000 miles per annum and is a mix of motorway and A-road. I've checked the onboard computer mpg by measuring the fuel added and by checking the mileage. I took the two-year-old one to a Ford garage and asked if they could check the poor mpg under warranty, but they said I must be mistaken and refused even to book it in. Obviously the lower mpg is costing me, but I'm more worried that this may be an indication of something sinister, like high friction of internal parts which may subsequently fail prematurely. Any suggestions?

Asked on 10 August 2010 by autowally

Answered by Honest John
The old one won't have a diesel particulate filter and the newer one possibly does.
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