I have a 59 plate 2 litre TDCi Titanium estate with 48,000 on the clock.
It gets serviced once a year in late Autumn as I do less than 12,000 miles per year. 16 months ago, after service, I noticed that the fuel consumption dropped to the low 40s MPG per tank, but after some months went back to its previous level of around 46-49 MPG per tank (and over 50 on long runs). This year exactly the same has happened, serviced about 4 months ago - poor fuel consumption ever since even on longish journeys. I can't get it above 43MPG however carefully I drive.
I can think of a few reasons, including

• The garage didn't reset something.
• New oil is thicker than 12,000 mile old oil - more drag.
• Winter weather is colder and damper.

Any ideas what might be the real reason?

 

Would of thought you mite use slightly more fuel in winter due to heaters lights and choke being used a lot more.

 

Thanks, but I shouldn't think so, because the drop in fuel consumption occurs immediately after the service not progressively as the weather gets colder and the nights draw in. It's not a gradual process.