An Italian consumer group is inviting car buyers to join a class action lawsuit against Fiat and Volkswagen over allegations that the companies exaggerated the fuel-saving credentials of their vehicles.

After conducting tests on Fiat’s Panda 1.2 and Volkswagen’s Golf 1.6 TDI models, Altroconsumo said on Tuesday the two models were between 18 and 50 percent less fuel efficient than stated by the two manufacturers in their marketing and sales material.

The consumer group is asking Fiat to pay compensation of €247 ($320) to each buyer of the Panda model and €509 to owners of the Golf model tested, based on what it estimates to be additional fuel costs for an average annual distance driven of 15,000 km (9,320 miles).

“Consumers buying supposedly efficient cars are misled too often,” Monique Goyens, director general of the European Consumer Organization, of which Altroconsumo is a member, said in a statement.

“The deplorable side-effect of this practice is that drivers might disregard fuel consumption information altogether.”

Altroconsumo alleged that Fiat and Volkswagen were using loopholes such as over-inflating tires or reducing the vehicle weight, giving significantly lower consumption figures than realized in real-life driving situations.

Officials at Fiat and Volkswagen both said they were unable to comment immediately.

Altroconsumo said the class action will go ahead regardless of how many people join and expects to deposit the papers with an Italian court within weeks.

EU regulators are already preparing draft legislation that will require a vehicle’s fuel consumption performance to be tested on roads rather than in laboratories.