Â鶹´«Ã½

Hillary Shaw
Newport, Shropshire, UK

There are around 1.7 billion motor vehicles in the world, from motorbikes to off-road large transporters (and this neglects some 1 billion pedal cycles, whose tyres are small). A minority of these vehicles have more than four tyres, some huge. So, let’s say there are the equivalent of 8 billion car tyres, which are, on average, 0.2 × 0.2 metres in cross-section and 1.6 m in circumference, and these tyres are typically inflated to 2 atmospheres. So, tyres account for about 1 cubic kilometre of air.

CO₂ is 1.5 times as dense as air, so to achieve the same tyre pressure would take 0.7 cubic km of the gas. However, Earth’s atmosphere has an “equivalent” volume of 4 million cubic km (if all its gases were at sea-level density) and has currently 427 parts per million CO₂, so that’s around 1700 km³ of CO₂ (at sea level density) in our atmosphere.

Worse, if you made drivers inflate their tyres with CO₂, they would have to drive more (to a filling point), and you would need an industry to extract, store and transport the gas. And as tyres lose pressure, their CO₂ would re-enter the atmosphere. Garages would love this; the planet would hate it.

John Davies
Lancaster, UK

Tyres are commonly inflated with compressed air, and for normal purposes, this is entirely adequate. But compressed air contains water vapour, which may condense into water in cold weather, or vaporise as the tyre heats up in use, changing the pressure. And oxygen gas is a smaller molecule than the nitrogen in air and can percolate through the tyre rubber so that it deflates, very slowly. Compressed air may be dried, but dry nitrogen is widely available stored in a cylinder and pure nitrogen gas will stay in the tyre longer. Formula 1 teams use dry nitrogen to inflate their tyres, so that the pressure will be maintained and predictable.

Carbon dioxide is also available in cylinders for carbonated drink machines that are small enough to carry in a bicycle saddlebag, yet with enough gas to reinflate a punctured tyre. But the carbon dioxide molecule is even smaller than the oxygen molecule and the tyre will deflate in a day or so.

Mike Clarke
Castle Hedingham, Essex, UK

Carbon dioxide offers no advantage over air, which is free and mostly readily available. Then, of course, there is the environmental impact of the inevitable leaks. I’m not sure how you get rid of all the air already in the tyre to start with either. I can’t think of any pros. Air seems to work quite well.

To answer this question – or ask a new one – email lastword@newscientist.com.

Questions should be scientific enquiries about everyday phenomena, and both questions and answers should be concise. We reserve the right to edit items for clarity and style. Please include a postal address, daytime telephone number and email address.

Â鶹´«Ã½ retains total editorial control over the published content and reserves all rights to reuse question and answer material that has been submitted by readers in any medium or in any format.

Terms and conditions apply.