Jamie Stone said people living in rural parts are facing far higher costs than others in more urban areas.
Liberal Democrat analysis shows households in rural areas can pay £114 a week in transport costs – nearly £40 more than those living in towns and cities.
Accusing the UK Government of “refusing to lift a finger to help”, Mr Stone demanded action from the five Tory MPs who are fighting to become the next prime minister.
The Liberal Democrats want rural fuel duty relief to be doubled to 10p a litre, as well as claiming the scheme should be “significantly expanded” to cover more areas.
The party also wants an emergency cut in VAT, reducing the charge from 20% to 17.5%.
According to the Liberal Democrats, these two measures combined would save the typical driver in a rural area £7.60 each time they fill up their car – meaning the changes could be worth almost £200 a year for those who fill up twice a month.
Mr Stone, who represents Caithness, Sutherland and Easter Ross in the north of Scotland, said: “If you live in a remote or rural area, you are utterly dependent on your car to get to work, access medical care or get your kids to school.”
He told how rural households are “bearing the brunt of devastating fuel price hikes, yet the UK Government is refusing to lift a finger to help”.
Mr Stone continued: “Scottish Liberal Democrats will continue to stand up for the residents of rural Scotland who have been badly let down by both the Conservatives and the SNP.
“All of the Tory MPs in the running to be the next prime minister should commit to helping rural families on the brink, by expanding the fuel duty relief scheme.”
Five MPs are currently in the running to succeed Boris Johnson as the leader of the Conservatives and become the next prime minister – Rishi Sunak, Tom Tugendhat, Liz Truss, Penny Mordaunt and Kemi Badenoch.