Why are green valve caps starting to appear on more car tyres in Spain?

More and more cars in Spain have small green caps on their tyre valves. At first glance they look like some sort of fad, but their presence reveals something much more interesting about how that tyre is inflated.

Although valve caps are one of the most commonly stolen items from cars - especially metal ones, due to their value and easy accessibility - these green ones are in fact a visual code that identifies an unusual type of inflation.

Vehicles fitted with them do not use conventional compressed air, but their tyres are inflated with nitrogen. While only a few cars have green caps, they are becoming more common due the technical advantages. But filling tyres with nitrogen instead of oxygen also has its disadvantages.

The main attraction of nitrogen is that it maintains pressure better. Nitrogen is less sensitive to temperature changes, so its volume varies less and the tyre stays inflated for longer.

In addition, nitrogen escapes more slowly due to the larger size of its molecules, so the tyres take longer to deflate. Another advantage is that it reduces the internal oxidation of the tyre and reduces the likelihood of fire, as it does not contain oxygen.

The main negative point is the lack of availability: you cannot fill a tyre with nitrogen at any old filling station and you would need to find a specialist mechanic or garage that offers it. Added to this is the price: nitrogen is more expensive than traditional inflation and usually costs between three and five euros per tyre.