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I wonder why a car has 4 wheels but has only one spare wheel. Is it that only one tyre can puncture at a time? If not, what if more than one tyre punctures on the go?

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    I'd argue in many cases ( compact city car ) you maybe don't need one at all. Given the space it takes and the good reliability of modern tires and the fact that half the population wouldn't change it on their own anyway.. (I mean that even without introducing run-flats) – agentp Sep 5 '17 at 15:00
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    Anyone else think that changing a flat should be part of the driving test? – Solar Mike Sep 5 '17 at 18:19
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    @SolarMike That's funny but also absurd. Just try pulling the lug nuts after some garage torqued them on at 50 ft-lbs. Further, that would rule out physically handicapped. To say nothing of the cars which do not even have a spare. – Carl Witthoft Sep 5 '17 at 19:32
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    when was the last time someone you know has needed a second spare wheel? – njzk2 Sep 6 '17 at 4:48
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    Having travelled the equivalent of 10-15 earth equators, I needed a spare tire exactly once. Spare wiper blades or spare batteries would have made more sense. – phresnel Sep 6 '17 at 9:31
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Many modern cars have no spare wheel at all. BMW have recently started implementing run-flat tyres across the range. Tyres which can, even with a puncture and no air pressure, be used at a reduced road speed.

Some other manufacturers have begun to include a compressed can of "tyre weld" or similar which attaches to the valve of a flat tyre and injects a sealant foam, again so the vehicle can be driven at a reduced road speed to a tyre bay for proper repair.

Carrying a spare wheel is quite an expensive business. It's expensive from a perspective of space, it's expensive from a perspective of weight and its expensive from the added production costs. However, generally a single spare is seen as a good compromise. There is nothing to stop a multiwheeled vehicle picking up more than one puncture at a time but this is an unlikely scenario.

Continent crossing vehicles (such as Dakar rally cars) will generally carry multiple spare wheels strapped to bonnets, roof bars or on the tailgate. This is because they may have to travel hundreds of miles before finding a garage.

For most consumer vehicles, they're usually never more than a few miles from a town or village with a tyre bay. Also, they're usually used on roads where busses operate and mobile phone coverage is good. If you were unlucky enough to have two punctures, the likelyhood is you can call a local garage, continue your journey on the bus or at worst case, have to flag down another passing car.

I should add that the most common reason why a four wheeled vehicle requires two tyre repairs is that a puncture has happened and the owner has found that the spare is also flat.

  • Cars on long distance rallies in remote locations are likely to carry an even bigger collection of inner tubes for quick puncture "repairs". In an emergency, you only really need a new tire if the casing is damaged badly - e.g. cut by a sharp rock, or damaged in an accident. An inner tube is enough to stop an air leak. – alephzero Sep 5 '17 at 15:12
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    @alephzero, if you're in a race, changing the whole tire is a lot faster than repairing a flat, especially if you have to unmount the tire and put in a tube. – JPhi1618 Sep 5 '17 at 16:15
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    Also you typically drive much more carefully and slowly when you're running a spare tire... So you would need to puncture 2 tires. Chances are, if 2 of your tires need replacing, so does the part of your car between those two tires. – corsiKa Sep 5 '17 at 20:40
  • Yeah I found my spare nearly flat when I first went to use it. I had a sharp word with my mechanic about that one. Free tire pressure check with oil change didn't include the spare. – Joshua Sep 6 '17 at 18:08
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    @corsiKa - That would be true in most civilized nations, but around these parts you see people running up the highway at speed on the things ... STU-PID! – Pᴀᴜʟsᴛᴇʀ2 Sep 7 '17 at 0:01
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Take a look at some classic antiques. Certain models have two spares, one mounted in each of the front quarter panels behind the axle.

I found approximately a zillion examples here

As tires, and roadbeds, got better designed, the probability of getting a flat (or more) rapidly moved asymptotically close to zero. Savings in cost and weight are critical to sales success, hence the disappearance of spares.

  • Many of those "zillion examples" show a vehicle with just one spare, either on the side or in the back, or else it's hard to tell from the image if there are two spares or just one. (I'm not saying you're wrong; I'm merely saying that the link by itself isn't all that convincing.) – user9527 Sep 5 '17 at 21:32
  • "Asymptotically close to zero"?! I've bought two new tyres in the last year. Roads in the UK are not good. – Andrew Leach Sep 5 '17 at 23:20
  • @AndrewLeach: For which total travel distance? – phresnel Sep 6 '17 at 9:32
  • @phresnel On that car, well under 3000 miles. Some major, government-maintained (rather than County-maintained) roads are literally falling apart. – Andrew Leach Sep 6 '17 at 9:33
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    @AndrewLeach you're a statistical outlier. I've had three punctures in ~200,000 miles on British roads. One of those I know was from a nail when parking on waste ground. In all cases the puncture was slow enough that I could drive to a tyre repair garage. – slim Sep 6 '17 at 13:05
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With modern tires on decent-quality roads, you can expect a flat tire about once every 150,000 km. So the chance of having a flat within one given trip is very small (let's say 1/1000 if your trips are 150 km long on average). The chance of having two flat tires at the same time independently of each other becomes 1/1,000,000.

That chance is so small, the cost of carrying a second spare with its drawbacks (weight ~25 kg, increased fuel consumption, and the space it takes up) far outweighs the benefits.

If you have two flats simultaneously, chances are you'll have 4 flats because you've driven over agressive one-way traffic enforcement measures or a police blockade. 2 spares aren't going to help you in those situations.

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