Some of the biggest names in the car industry have denied cheating emissions tests in the wake of the Volkswagen emissions scandal.
The Which? consumer group asked 19 firms about whether they had been misleading the public by manipulating tests.
Renault, PSA Peugeot Citroen, Nissan, Hyundai, Jaguar Land Rover, Kia, Mazda, Tesla, Volvo, General Motor Company, Honda, BMW, Ford, Toyota, Suzuki, Daimler and Mitsubishi all denied manipulating tests while Fiat Chrysler and Subaru did not respond within the seven-day deadline.
VW has admitted that 11 million of its diesel vehicles worldwide were fitted with defeat device software which conned testers into believing their vehicles met environmental standards.
The public remains "suspicious" despite these denials and there are still questions about the effectiveness of testing, both on emissions and on fuel efficiency, according to Which? which is calling for an independent investigation.
It is therefore important for the Government to immediately publish a timetable for a genuinely independent investigation and ensure any consumers found to be affected can get easy access to redress, it said.
The European Commission is planning to implement the real driving emissions procedure which would mean that new cars will have to be tested both in the laboratory and on the road.
The new worldwide harmonised light vehicles test procedure (WLTP) could also bring improvements, according to Which?, and see more realistic CO2 and fuel consumption figures in place.
Daimler and PSA Peugeot Citroen stressed they support the introduction of the WLTP from September 2017.
Renault Group said it backs the implementation of European homologation tests which aim to be representative of real conditions.
Which? executive director Richard Lloyd said: "It's reassuring that the majority of car manufacturers have explicitly denied rigging emissions and fuel economy tests. However people will rightly be suspicious of these tests until an independent investigation has been carried out.
"The Government must now be clear with consumers about the action it is taking, including real life testing and redress for people who have been misled."
In its newly-launched Come Clean On Fuel Claims campaign Which? is calling for all car manufacturers to announce whether their emissions and fuel economy tests have been manipulated. It also wants the Government to publish a timetable for a independent investigation and ensure any consumers found to be affected can get easy access to redress.
It is also calling on the European Commission to announce new tests, which come complete with independent supervision, by the end of the year.
VW's new chief executive has told more than 20,000 gathered workers that the company will have to review planned investments and contain costs as it works to overcome a scandal over vehicles that cheated on emissions tests.
Matthias Mueller told the meeting at the company's sprawling home plant in Wolfsburg, Germany, that VW would have to put its future investments in plants, technology and vehicles "under scrutiny" to spend only what was needed to maintain a leading edge.
Volkswagen faces fines and lost sales after US environmental regulators found it had installed software that disabled pollution controls when the vehicle was not on the testing stand.
The company has set aside 6.5 billion euro (£4.8 billion) to cover costs but analysts say that is unlikely to be enough.
Mr Mueller said some of the cars could be fixed by adjusting the software, while others would need mechanical fixes.