Sometimes, car maintenance can feel like a full-time job. You’ve got to keep your engine oiled, your washer fluid filled, and your spark plugs… sparky? The list goes on. And underpinning all of them are four of the most finicky bastards you ever came across: your tires.

Oh, they have too much air in the summer; and too little in the winter; they can blow out randomly on the highway sending you face-first into the hard shoulder if you’re lucky – the problems are never-ending. Which might make you wonder: why do we even bother having air in our tires at all?

It’s not quite as silly a question as it sounds. After all, there are vehicles that don’t have air-filled tires: tanks, for example, or industrial vehicles like forklift trucks work perfectly fine without them. But when it comes to our cars, trucks, buses, and even airplanes, we stick with the tried-and-true-enough combination of rubber and a very particular amount of air.

Well, it turns out there’s a very good reason for that.

The first pneumatic tires

Considering how long we’ve had the wheel, the idea of dressing them in puffy outerwear – side note: this is actually where the word “tire” comes from; it’s short for the wheel’s “attire” – is surprisingly recent. 

The first patent for a pneumatic tire – that is, one filled with air – was filed in the UK in 1845, by Robert William Thomson, of Middlesex, England. With his design, patented in the US in 1847, he wrote, “the wheels will in every part of their revolution present a cushion of air to the ground or rail or track on which they run.”

“I […] have invented or discovered a new and useful Improvement in Carriage-Wheels, which is also applicable to other rolling bodies,” he boasted. By “the application of elastic bearings round the tires of the wheels,” he wrote – the tires, in this case, referring to the standard solid steel or rubber rims that would be welded to the outside of the wheels – the effect would be a “lessening [of] the power required to draw the carriages, rendering their motion easier, and diminishing the noise they make when in motion.”

Of course, this was long before the invention of synthetic rubber, which makes up the majority of rubber used in the tire industry today, so Thomson recommends using “sulphurized caoutchouc or gutta-percha, and inflating it with air” for his design. But the idea is basically there: get some rubber, or a rubber-like substance, fill it with air, and let your butt thank you later.

As inventions go, it was pretty much the holy grail: an easy, cheap tweak that would provide major improvements to people’s everyday lives. This is why it’s so strange that nothing came of it for decades – and in fact, it would take an entirely different inventor to actually get the tire game rolling.

The quest for comfort

We might not know what inspired Thomson, but when it came to the next big name in the history of pneumatic tires, John Boyd Dunlop – yes, that Dunlop – his motive for filling rubber tubes with air and slapping them around some wheels was clear: it was for his kid.

“John Boyd Dunlop was a Scottish veterinarian who had relocated from Scotland to Belfast,” wrote cycling journalist and author Suze Clemitson in her 2017 book A History of Cycling in 100 Objects. “Watching his son labor over the cobbles on his tricycle he was inspired to invent – or re-invent – the pneumatic tire in a bid, as reports variously suggest, to save his son from constant headaches or a sore behind.”

Like Thomson, his design used rubber treated with sulfur – a process developed in 1844 by Charles Goodyear, and now known as vulcanization – but unlike Thomson, Dunlop’s tires immediately took off. Why? For two main reasons: firstly, he actually produced and sold them, which definitely helps if you want to be commercially successful. But he also had a hefty dose of luck on his side: “The bicycle boom was at its height when Irish cycling champion Willie Hume purchased a set of Dunlops for his bike the following year,” Clemitson explained. Hume became "the first ever rider to use pneumatic tires in competition and, it's said, never [lost] a race when riding on them.”

So, intra-Scot inventor drama aside, we can already see two apparent advantages of using air-inflated tires over their solid predecessors: speed, and comfort. But why should that be the case?

Why we fill tires with air

We know what you’re thinking: what an easy question, right? It’s the same reason we fill bouncy castles with air instead of letting kids jump about on big old lumps of steel and polymerized rubber.

Aside from the obvious, there’s some pretty cool physics going on. As a gas, air can be compressed far more than any solid material – which is important, when you’re one of just four wheels bearing the weight of a 1,600-kilogram-or-so car. 

Even lighter vehicles, like bikes, will inevitably deform the wheel slightly – which is a good thing, because it increases the amount of wheel on the road at any one time, in turn providing more traction for the vehicle – and it takes a lot less energy to do that with a tire filled with air than a solid one. This is why early adopters of Dunlop’s bicycle tires were so much faster than their competitors: they were easier to maneuver at higher speeds, for a lower energy cost.

In physics terms, this is called the “rolling resistance”: the energy consumed by one tire per unit distance covered. To put it simply, a lower rolling resistance is better – and here, air-filled tires have the advantage over solid ones. 

That’s not just due to their malleability. It’s also because air is, well, lighter than solid rubber: “for the same deformation, the more massive an object is, the more it heats up,” explains Michelin, and that heat loss translates to a higher rolling resistance.

And all of these advantages are thrown into especially sharp relief when you’re trundling down the road at 60 miles per hour (97 kilometers per hour) and a stray pothole jumps out at you from nowhere. At those speeds, a sudden sharp obstacle would cause a huge jolt to a solid wheel, which could only absorb the shock locally; air, on the other hand, would dissipate the impact across the entire wheel, making for a smoother ride. 

Like we said: your butt will thank you later.

Why don’t we fill tires with something else?

So, we’ve figured out why gas-filled tires are better for most everyday purposes than solid ones – but should that gas necessarily be air?

Well, if you ask some drivers – like a Formula 1 racer, for example – they’d say no. While air has many advantages, there are also a few drawbacks to filling your tires with it: it will gradually permeate through the rubber, for one thing, and it’s relatively sensitive to heat and moisture changes. That’s why we need to adjust it throughout the year – if the temperature outside drops by 10°C (18°F), say, the pressure in your tires can drop in response by up to 0.14 bars (2 PSI).

Now, for most of us, this really doesn’t mean much. But if you’re competing all over the world at speeds of 200 miles per hour (322 kilometers per hour), there is a more high-tech option: fill your tires with nitrogen.

As a “dry” gas, nitrogen removes the problem of moisture in your tires, making the pressure and compression of the tires more consistent. At the speeds those racers go at, those details can be life-saving – as anyone who’s ever had a blowout at a meager 60 or 70 mph (97 or 113 kmph) can only imagine.

Like many high-octane tech modifications, the idea of nitrogen-filled tires is slowly spreading outside of the professional world and onto the highways. So, should you go out and drop your paycheck on a set of four N₂ tires? 

Eh, probably not.

“It’s true that there is a slower loss from nitrogen-filled tires,” notes tire retailer Les Schwab. “But this improvement is slight […] It’s not enough to make a true difference in gas mileage or tire wear for people driving passenger vehicles.”

And that makes sense. Air is already mostly nitrogen, after all – it’s about 78 percent nitrogen to just 21 percent oxygen, in fact – and the nitrogen you’d end up filling your tires with will top out at around 95 percent N₂. “It’s never 100 percent,” Les Schwab says.

“Bottom line: Nitrogen will slow the amount of tire inflation loss to about one-third of what you’ll experience with air,” they write. But “you’ll still need to check and top off your air roughly every other month to stay within the ideal inflation range."

“And you’ll spend far more than you’ll save on gas and tire tread life,” they add. “You’re better off making simple tire maintenance part of your routine.”

All “explainer” articles are confirmed by fact checkers to be correct at time of publishing. Text, images, and links may be edited, removed, or added to at a later date to keep information current.