Why is my Audi A6 TDI's fuel economy so poor?

The fuel economy of my 2007 Audi A6 2.0TDI Avant Multitronic has always been poor from new at 34mpg, rising to 40 in mid 2011 and now having dropped steadily back to 33mpg. Most of my mileage consists of short runs of up to 30 miles but, even on annual trips to Europe of 4000 miles or more, the mpg does not change appreciably.

I seem to have narrowed down the cause of the low mpg to the DPF and possibly also the speed sensor in the gearbox. Is the removal of the DPF a sensible course of action? The gearbox sensor would cost about £1000, although it would eliminate the engine doing 4000rpm before changing up.

Asked on 28 April 2012 by RN, via email

Answered by Honest John
I've had reports of 55mpg from A6 2.0TDIs run by airline pilots, almost all on the motorway. What might be happening is that you are running at insufficient revs to properly trigger active regeneration of the DPF. If the engine oil level rises from contamination from fuel, then that is the reason, but you'll get an idea of the true average mpg here: www.honestjohn.co.uk/realmpg.

You are not allowed to remove the DPF. But, interestingly, in the USA, manufacturers are compelled to offer an eight-year warranty on DPFs. There is now a VAG TSB recall on the injectors for your engine and that might solve the problem.
Similar questions
I'm currently running a 2009 Volkswagen Golf Plus 2.0 TDI that I've had for almost five trouble-free years. It'll be at 90,000 miles soon and I think I should get out before I have to spend money on it....
We have a Volkswagen Golf 1.6 TDI which has had the software fix applied. Over the last month or so we've had a number of trips to London covering A and B roads using premium fuel and within the speed...
I recently purchased a 2015 Vauxhall Astra diesel 1.6 CDTi, attracted by the high quoted MPG of 76 (combined). I always calculate the cars MPG when I fill up, as well as recording the figure the car says....
Related models
High quality and refined cabin. Good engines including frugal TDI diesels. Maximum five-star crash test rating. Classy image.

Ask Honest John

Value my car