Audi-maker Volkswagen has been ordered to qualify its advertised fuel consumption figures to reflect that they might not match the reality of "real" driving by consumers.
Audi's website claimed that its A3 TDI was "the most fuel efficient Audi ever" by achieving a "quite remarkable" 68.9 miles per gallon (mpg).
But one driver who bought the car said the figure was misleading and challenged if it could be substantiated after being unable to achieve the same result.
Defending the claim, Volkswagen (VW) said the figure was obtained from the manufacturer's tests carried out in accordance with EU regulations, but said complaints from customers who had been unable to achieve the same result were "occasionally encountered".
It said fuel figures did not give an accurate representation of the actual consumption that could be expected from any particular vehicle and were provided "only to enable comparisons between different vehicles or models".
There were also "infinite variations" in driving styles and in road, car and weather conditions, all of which could have a bearing on the results.
While it understood that such figures had to be generated under test conditions so consumers could compare them on a like-for-like basis, it believed it was unlikely that the average consumer would realise it was not necessarily representative of their own driving.
The ASA said: "For that reason we considered that VW should have qualified the figure to make clear to readers that it was based on an EU test for comparative purposes and may not reflect real driving results. Because they had not done so we concluded that the claim breached the code."
It ruled: "We told VW to ensure they qualified official fuel consumption figures to make clear that they were official EU test figures to be used as a guide for comparative purposes and may not reflect real driving results."