Many of the top car hire companies are failing to disclose extra costs, making it "almost impossible" for consumers to find the best deal, according to a watchdog.
Which? tried to rent a car in Spain from 10 major hire companies and broker websites, but even at the end of the booking process 53% of its researchers were not satisfied that they knew the total price.
The investigation also found 75% of those who had to pay a compulsory cost for a full tank of fuel did not know how much they would need to pay when they picked up the car.
More than one in five (22%) did not know the amount of excess and 44% did not know the cost of the optional excess waiver to reduce the amount they would have to pay if the car was damaged.
Such payments can add more than £100 to the cost of a week's rental.
More than half (54%) did not know if the excess waiver covered them for damage to tyres, the windscreen or underneath the vehicle.
Which? is calling for all car hire fees to be made clear to consumers upfront and before they make a booking, saying that it is a principle of European and UK law that essential information such as compulsory and other significant charges must be clearly stated at the point of purchase.
Smaller companies topped a customer satisfaction survey of car hire firms by the consumer group, with Canary Islands-based Auto Reisen beating more well-known names with a score of 89%.
Goldcar was bottom of the table with a customer score of 42%.
Thrifty and Enterprise took second and third place respectively, while Europcar and Budget rounded out the bottom three.
Economy Car Hire topped the broker table of ratings with 78%, with Argus Car Hire at the bottom scoring 52%.
Which? said it would be sharing details of its investigation with the companies to encourage them to make improvements, and had also written to Avis, Dollar and Enterprise "to remind them of their legal obligations".
It is calling for all car hire companies and brokers to clearly show all the fees and extra charges on their websites during the booking process, including the amount of excess, the cost of excess waiver and what it covers, the cost of compulsory fuel and the company's fuel policy as well as the cost of all optional extras like satnavs and additional drivers.
Which? executive director, Richard Lloyd, said: "The car hire industry is taking customers for a ride by hitting them with sneaky charges not included in the headline price.
"Not being upfront about the total cost makes it almost impossible to shop around for the best deal and leaves people on tight budgets paying more than they planned. Car hire companies must be more transparent and upfront about their fees so people can make an informed choice."