A nationwide analysis of petrol and diesel prices found Donegal to be the most expensive county in which to filling a tank of petrol, while Wicklow was the dearest for diesel.

The average cost of petrol in Donegal was 128.1 cent per litre, compared to 122.7c in Mayo, which was the cheapest county for both petrol and diesel. With more than 5c in the difference between highest and lowest, this amounts to a difference of more than €3 in the cost of filling a tank, based on an average tank size of 60L.

The cost difference is even more stark for diesel. In Wicklow, diesel costs on average 116.3c per litre, while in Mayo and Roscommon, the cost is 106.5, a difference of almost 10c. Based on the cost of filling a 60L tank, the motorist leaves the forecourt nearly €6 better off in Mayo than in Wicklow.

The national average cost of petrol is 126.5c per litre, which is also the average price in Dublin. However, diesel in the capital, at 115.6c, costs above the national average of 112.8c per litre.

In Cork, the cost of petrol is slightly above the national average at 126.9c per litre, while diesel is lower, at 112.1c. Kerry is below the national average for both petrol and diesel, at 123.9c and 108.7c respectively.

The AA found a number of garages in border counties to be charging the highest in the Republic, with rates of 131.9c per litre, 5.4c above the national average. The AA’s director of consumer affairs, Conor Faughnan, said that border counties tend to be more expensive places to fuel up by virtue of geography.

“It is common for motorists to cross the border in an effort to dodge the UK’s considerably higher pump prices,” said Mr Faughnan. “That, coupled with its rural nature, make it unsurprising that Donegal is a relatively expensive county. It is certainly worth shopping around for the best deal as there is a significant difference of 7.1c in the same county.”

Mr Faughnan said there were garages in both Donegal and Wicklow where cheaper prices can be found.

“The message to drivers is to be canny about this and to shop around,” he said.

Laois was the second-most expensive county for petrol, while Wicklow ranked third.

Mayo and Roscommon are the cheapest counties to fill a diesel car, with two service stations in the respective counties selling for “a very impressive” 99.9c a litre.

Mr Faughnan said there was “quite a bit of variation and we should be careful about reading too much into it”.

The AA’s National Fuel Price Index tracks monthly national average figures, with petrol now costing 5.1c less per litre compared to the same time last year. Diesel also dropped this month and now averages 112.8c.

While all motorists are benefiting from the fall in crude oil prices internationally, pump prices are still not as low as they were the last time oil fell to these levels, because of higher Irish taxes, Mr Faughnan said.

Fuel tax in Ireland is charged by litre, so when the price falls, the tax does not.