The normal air that you fill in your tyres consists of 78% nitrogen, 21% oxygen and 1% other gases and water. In comparison to compressed air, which produces a by-product of water, a nitrogen-filled tyre has less water vapour. That's why, at high temperatures, compressed air in a tyre expands at an unexpected rate. But a nitrogen-filled tyre will expand at a much slower rate due to the absence of water content.