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I have a long commute, so I spend a lot of time staring at the cars ahead of me. One unusual thing I've observed is that on most cars the left-hand side tires look like they have less air than the right-hand side tires.

Why is this?

Hint: cars drive on the right side of the road where I live.

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    $\begingroup$ rot13(Yvar-bs-fvtug + crefcrpgvir), perhaps? $\endgroup$ – Avi Dec 20 '19 at 22:30
  • $\begingroup$ @Avi you should make that an answer! $\endgroup$ – 2012rcampion Dec 20 '19 at 22:32
  • $\begingroup$ Unfortunately, I'm going to be on my own long commute momentarily, so I'll have to leave that up to someone else. $\endgroup$ – Avi Dec 20 '19 at 22:34
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Your “long commute” is actually

an automobile race on an oval track. Left tires are slightly smaller than right tires because it makes it easier to always turn left.

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  • $\begingroup$ Love the idea, but doesn't jive too well with "cars drive on the right side of the road" $\endgroup$ – Bass Dec 22 '19 at 4:12
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    $\begingroup$ Although I do drive in a big circle, I'm talking about ordinary cars. That's a very cool fact though! $\endgroup$ – 2012rcampion Dec 24 '19 at 19:05
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I am taking you have stout drivers in our area that do not take their colleagues to work with them.

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    $\begingroup$ I thought of that too, but even a 300 lbs person in a car weighing, say, 3 tons only increases the weight on one side by around 10%. This isn't a large enough difference to cause the effect, which can be seen in cars of all sizes. $\endgroup$ – 2012rcampion Dec 24 '19 at 19:10
  • $\begingroup$ 10% more weight make the flat spot at the bottom 10% larger (because air pressure in the tire counteracts weight) yeah this is probably not visible $\endgroup$ – Jasen Dec 29 '19 at 0:09
  • $\begingroup$ A 3 ton passenger car is an extremely rare sight in my part of the world; even 2 tons cars are difficult to find. Then again I have never seen a 300lbs person outside the US either, so perhaps your back-of-the-envelope calculation has some value after all. $\endgroup$ – Servaes Jan 22 at 1:41
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Avi and DEEM came close to the right answer; the effect is actually

an optical illusion.

It's caused by a combination of two factors: one,

the bulges on the sides of a flat or slightly deflated tire are only visible when you are directly in line with it.

enter image description here

Two,

the tire on the driver's side of the car ahead of you is nearly aligned with your line of sight, but the passenger side is viewed at slight angle. In a left-hand-drive country this makes the left-hand tire appear flatter.
enter image description here

Surprisingly the combination of these two factors is enough for the effect to be clearly visible.

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  • $\begingroup$ Interesting. Given this is the intended answer, though, I'm not sure the "lateral thinking" tag actually applies and was probably throwing people off... $\endgroup$ – tmpearce Jan 18 at 4:34
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Several optical things can result in this illusion.

First, most roads are slightly elevated in the middle so the driver side (in US for example) is slightly elevated

Secondly, windshields are slightly curved. So looking straight at the left side gives a different perspective than looking at the right side tires

Thirdly, if you draw a line (of vision), as a driver, the left side tires in front of you are at a shorter distance than the right side tires

So in combination an illusion is created

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  • $\begingroup$ I don't think either of the first two result in any noticeable difference, but the third one is close. Whether the tire is slightly higher, lower, closer, or further doesn't affect how flat it looks, but something else does. $\endgroup$ – 2012rcampion Dec 24 '19 at 19:13
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Maybe it's because of

Temperature. If you drive north in the morning and the return back to south in the afternoon, the sun will be shining the cars on their right side, thus increasing the temperature of the air, thus the pressure in the right side tires. This effect might only be noticeable for long traveling times.

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  • $\begingroup$ That's an interesting idea, I hadn't thought of that! But I notice the effect regardless of the direction I'm driving or the time of year so I don't think it has to do with the sun angle. $\endgroup$ – 2012rcampion Dec 26 '19 at 5:23
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One more answer (though @Avi's seem to have been accepted).

If you take the train and look out the left-hand windows, you may see the cars of a train coming from the opposite direction. Many trains (where I live) are next to roads, so if you look out the right side you may see automobiles. Train tires have much less air in them than automobiles :)

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    $\begingroup$ Since it's a lateral-thinking question, and this is sneakily clever, I'm afraid that I can't help but smack it with a +1 :) $\endgroup$ – Avi Jan 17 at 21:52

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