The cost of filling up a family car with diesel has hit £100 as the war in Iran sends fuel prices soaring, new figures show. It takes the cost of filling up a family car with petrol to £89. Ms Fahnbulleh said: "People are under huge pressure with the cost of energy, with the cost of fuel. However, Mr Williams said that the recent declines in oil prices should soon be passed on to motorists. High fuel prices bring new pressure on John Healey, the Chancellor, to delay a planned increase in fuel duty.