Nitrogen is an inert, non-flammable gas whose molecules are larger than those of oxygen. This means nitrogen diffuses through tyre walls three to four times more slowly than oxygen, so a tyre filled with pure nitrogen should maintain its correct pressure for three to four times longer than one filled with air (78% nitrogen, 21% oxygen, 1% other gases).
This boosts a vehicle's fuel economy by minimising tyres' resistance, prolonging life and delivering optimum grip. Simon Roper, managing director of Kwik-Fit Fleet, said: 'Apart from the fuel saving element, it should mean less time spent on tyre maintenance and checks.' He insisted that filling tyres with nitrogen in no way removed the responsibility of drivers to conduct regular tyre pressure and condition checks.