Nitrogen versus air in tyres

Shop air is already about 78% nitrogen. A nitrogen fill raises that fraction and removes most of the water vapour. The practical effects are modest: slightly slower pressure loss, and less moisture to corrode rims from the inside.

Nitrogen does not exempt you from temperature physics. A 10 °C overnight drop still takes pressure off a nitrogen tyre.

Aircraft use nitrogen for fire safety around hot brakes. A Focus in a Tesco car park is not that environment. If a dealer offers nitrogen with a clear recap policy, it is harmless. If they imply air is unsafe, walk.

Mixing is fine. Topping a nitrogen tyre with air slowly dilutes the fraction. Keep the placard. Keep the gauge.

A slow leak at the bead or a nail does not care which gas is escaping.