Why doesn't my friend's SEAT ST achieve its quoted fuel economy of 80mpg?

I have a friend who lives near Stirling, three miles from the nearest main road. He does not receive the Telegraph and so missed out on your wisdom and replies regarding cars not meeting their advertised fuel economy. Having owned Golfs for over 30 years he decided to go for better economy as he approaches retirement and bought a SEAT ST 1.2 TDI SE Ecomotive from Arnold Clark in Glasgow last December. He says the advertised fuel consumption is something like 80 mpg but he is only achieving 57.5 over 6,000 miles with little urban usage. I responded with your usual advice, but he went on to say that his wife has a SEAT Ibiza TDI Ecomotive that has a slightly larger engine and one more cylinder but achieves 75 mpg.

Also he thinks there is something fundamentally wrong with his car. The stop-start device only works intermittently. When he complained about the fuel consumption Arnold Clark fitted a new engine control unit, but that did not solve either problem. He wrote to SEAT and they responded saying that the records showed the engine control unit had been changed because of the stop-start device, not fuel consumption, and he was advised to take the car to his nearest dealer, which is Morrisons at Bannockburn. They refused to take it. Do you think he has a problem? If so, what should he do next?

Asked on 12 June 2011 by DC, Knutsford

Answered by Honest John
There's probably nothing wrong with it and 57.5mpg may be all he is going to get. The reason is the extra fuel used by the engine to burn off soot in the diesel particulate filter. This consumption is not taken into account in the EC Certification tests. 80mpg is ludicrous pie in the sky that can only be achieved in the lab or by applying extreme economy driving techniques, as one of our guys did, in a Polo with the same engine. That way he got 84. Normally he gets 61. I don't believe that your friend's wife actually achieves a regular genuine 75mpg.
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