The main supplier of vehicles, TrustFord Guernsey, has announced that if the duty is implemented, it would withdraw any vehicle larger than a Fiesta from the fleet.
And hire car companies believe that this will not only affect their business, but also the tourism industry.
Mark Finn, a director of Value Rent A Car, which also holds the licence for Avis, has sent a list of questions to deputies about the proposals.
'This tax does affect our company in the type of vehicles that we have on our fleet, but who it really affects the mostly are our customers who are going to be severely restricted in the type of vehicles that are available to them,' he said.
Mr Finn said vehicles larger than a Fiesta accounted for 75% of his fleet, but it was the other 25% that was vital for the running of a car rental company.
'At present, no other motor manufacturer is offering terms to obtain/lease vehicles and service from them that we require.'
Hired cars are subject to an exemption that would see them have to pay only 15% of the duty, but this would have to be claimed back and would only be in cases where the car was exported after use.
