NHS Highland is being urged to review the allowance it pays for patients in the islands or rural areas who have to travel more than 30 miles.
The health board's car mileage rate has been cut from 18p per mile to 13p.
The Scottish government allows health boards to set their own mileage rates for patients using a car to get treatment.
But there is concern at the widening gap between health boards responsible for the Highlands and Islands.
NHS Highland, which includes the islands of Argyll and Bute Council, has cut its mileage rate to 13p whereas NHS Western Isles pays 24p per mile.
Government guidance
Critics said the impact on patients was exacerbated by an almost 30p a litre difference in fuel prices when comparing Lewis and islands such as Colonsay.
Ian Gillies from Tiree - who is a former chairman of the Scottish Islands Federation - said NHS Highland should consult on the issue.
The health board said its rate was based on the Inland Revenue's advisory fuel rates and Scottish government guidance.
The NHS Highland mileage rates apply to patients who have to travel more than 30 miles - or more than five miles by water - to get to hospital.
Patients can claim repayment of travel expenses without a test of means, minus the first £10.00.
