Of the three countries, Spain is the cheapest place to fill up, and for both fuels. Median prices on 3 November 2026: diesel in Spain at €1.929/L against €2.249/L in France and €2.289/L in Portugal; petrol 95 at €1.739/L against €2.089/L and €2.064/L. But a price gap in cents is not yet a saving. The site calculates a 50-litre fill-up minus the fuel burned on the detour there and back at 6.5 L/100 km, and calls the trip “Worth the drive” if you keep €10 or more with a detour of no more than 45 minutes. Time, wear and tolls are left out of the calculation.
From Portugal to Spain
Right at the border the saving looks obvious, but the detour eats into it: of the 101 concelhos (municipalities) within 50 km of the border, a full tank in Spain pays off in just 15 (prices for 3 November 2026). The calculation for each one is on the border page.
The French motorway
At motorway services, the median diesel price is 18 c/L higher than at the best filling station by the exit; worked out for 309 of the 409 services. So turning off the motorway for a filling station at the exit is often the better bet.