The advertising watchdog has warned carmakers to ensure they stipulate that mpg figures are from official European tests, and may not reflect real driving results, to avoid misleading consumers.
It issued the ruling following a complaint about a VW Group UK advert for an Audi A3 1.6TDI which stated: "The A3 1.6 TDI is the most fuel-efficient Audi ever returning a quite remarkable 68.9mpg on a combined cycle".
The Advertising Standards Authority said it received a complaint from a consumer who had bought the car, but found they were unable to achieve similar fuel economy.
It ruled the advert was misleading.
The ASA said: "We considered that it was unlikely to be clear to the average consumer that the figure quoted was based on a standardised test and was not necessarily representative of what they would achieve when driving the car themselves.
"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."