The previous rule of thumb was that filling up at a city supermarket was cheapest, with prices progressively increasing the further away you drove from urban areas.
That was certainly true on past annual family holidays to the north-west Highlands, where, after a certain point, there are no prices displayed outside filling stations, you just had to peer at the numbers on the pump – and probably gulp too.
But that wasn’t the case on this latest trip.
Instead of prices going up as we travelled into the wilds, they kept coming down.
Cheapest of all was a garage in Lochgilphead whose petrol was around 10p a litre cheaper than the forecourts we had passed leaving Glasgow.
Ironically, it was the second filling station we drove past in the mid-Argyll town – just after filling up at the first one.
The one we had stopped at – not realising Lochgilphead would have two – was slightly more expensive, but still cheaper than city prices.
Confirmation of this phenomenon came on our way home, when we noticed a supermarket filling station in Dumbarton charging significantly more.
So what’s going on?
According to the AA motoring group, supermarkets have abandoned cheap fuel as a loss leader to lure car-based shoppers, switching their discounting to products on the shelves, which is in turn offset by higher fuel prices.
In the past, I remember being sent regular announcements from Asda – traditionally the cheapest supermarket petrol – about their latest price cuts (never about increases), but those appear to have ceased some time ago.
The AA said small fuel stations were "stealing a march” on their bigger rivals by being the ones to pass on the recent big drop in wholesale prices, particularly petrol.
Fellow motoring group the RAC said it had also noticed smaller forecourts “prepared to buck the national forecourt trend”.
Rural stations like the ones we saw appear to be being more honest with motorists.
The AA said that while wholesale petrol was down 20p a litre compared to its July 1 peak, pump prices had fallen by an average of only 5.5p.
But it said a growing number of small independent forecourts were last week selling fuel as much as 10p a litre below the UK average.
By contrast, it said other fuel stations near supermarkets, who may previously have had to keep their prices down to stay competitive, were now happy just to match the higher superstore prices.
In rural areas, filling stations may be counting on extra business from summer holiday traffic, especially with the continuing boom in staycations.
Those which now have shops, including chain store outlets like Spar and Tesco Express, may also rely less on fuel sales for their profit, and could even be imitating past supermarket practice of using price to attract more customers through the doors.
As we found to our cost, pre-planning is needed to locate the cheapest fuel.
Price comparison websites exist, but the UK’s Competition & Markets Authority and the AA want a non-commercial scheme launched, like in Northern Ireland.
We’ll certainly do our homework next time.