When it comes to fuel economy in the UK as measured by the EU test cycle, what you see is not what you get.

According to tests performed by independent emissions testing company Emissions Analytics, most car models in the UK are failing to meet official fuel consumption figures.

Emissions Analytics’ chief executive Nick Molden reports that only eight of the 550 models tested by the company met its reported figures. According to Molden, the discrepancy results from lax testing practices that fail to duplicate real-world conditions.

In the U.S. test cycle, for example, cars are required to accelerate faster and undergo cold starts—neither of which are required in the EU test cycle.

Overstating fuel economy performance also comes with a higher price tag in the U.S., where Hyundai and Kia recently agreed to pay $300 million in fines for inaccurately reporting fuel economy and violating the Clean Air Act.

According to Brussels-based think tank Transport Environment (TE), the discrepancy between actual fuel economy and reported fuel economy in the EU rose by 23 percent between 2001 and 2013, and not by accident. TE asserts that carmakers intentionally influence the results of the testing by overinflating tires, disconnecting the battery, and taping over gaps in the car body.

Garel Rhys, emeritus professor of motor industry economics at Cardiff Business School, said European consumers should be concerned by the behavior of automakers and should start holding them accountable.

“[The carmakers] have to be shamed into changing – shamed in such a way that if they don’t, it could start affecting their marketplace,” Rhys told the Financial Times.

While both the UK Department of Transport and the UK trade body agree that the EU testing process is outdated, change has been slow coming. Carmakers continue to resist stricter testing and are currently lobbying to delay the EU’s efforts to implement a stricter testing procedure in 2017.