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I had imagined that run-flat tires are only available on 17" to 19" rims with particularly thin (aspect ratio-wise) tires. I thought they are engineered by building a slightly thicker rim-facing section, on which the rim would rest when the tire is flat.

But run-flat tires are also available on 16" rims, and they appear to be engineered by adding either circular sections radially or a matrix of rubber to partially fill the void.

Now that my first assumption is gone, I'd like to confirm whether either of my two other assumptions is also incorrect.

Which of the following is not true:

  1. Run-flat tires contain more mass, and the added rotational inertia would make it ever so slightly harder to accelerate and brake, not to mention that they would consume more gas. Hence you should install them only if you drive a large engine with power to spare, and you don't mind the extra expense and ecological damage.
  2. It is pointless to install run-flat tires on a car that will be used mostly for city driving, since their "run-flat feature" is only effective for high-speed/highway driving.

(No, no.. these are not two questions. They're one comprehensive question about one problem with more than one facet. I suspect your answer would be comprehensive as well, shedding light on both issues simultaneously.)

  • "run flats" are available in many sizes... – Solar Mike Jun 17 at 14:23
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Run-flat tires contain more mass, and the added inertia would make it ever so slightly harder to accelerate and brake, not to mention that they would consume more gas. Hence you should install them only if you drive a large engine with power to spare, and you don't mind the extra expense and ecological damage.

Yes they are slightly heavier - although this is generally more than offset by cars equipped with run-flats not carrying a spare tire. So typically you'll be having less weight and consequently better economy and less environmental damage etc. Plenty of low-powered cars out there on run-flats.

It is pointless to install run-flat tires on a car that will be used mostly for city driving, since their "run-flat feature" is only effective for high-speed/highway driving

No - certainly one of the benefits of them is more applicable to "high-speed/highway" driving as you put it. That being that they reduce the impact of a puncture on the driver's control of the car.

But there are other benefits too:

a) No need to stop and change the wheel at the roadside - which can be extremely hazardous even in non-highway scenarios. And of course doing so is time consuming and difficult for people who may be frail or physically impaired. Additionally depending on climate it can be downright unpleasant (I've changed wheels in driving rain, snow etc)

b) Not having to make room for a spare wheel means car designers can give more room in the boot - more luggage space.

Personally I'm not the greatest fan of run-flats, they are expensive to purchase, the extra re-enforcement on the side wall makes cars shod with them prone to crashy, uncomfortable rides, and they can't be repaired in the event of a puncture (see expensive to purchase). Worst of all if you suffer damage to the tire in excess of what the can take and remain functional then you're stranded and need recovery because chances are you don't have a spare wheel to swap to. But they do have their benefits!

  • The overall weight may be lighter but you're increasing your unsprung weight. – Steve Matthews Jun 17 at 15:38
  • @SteveMatthews Indeed - it's not a huge amount but it does impact on handling (and to a lesser degree ride). Which is another reason I'm not keen on them personally. The OP was looking at weight from an economy point of view which is why I kept that mindset when considering it here. – motosubatsu Jun 17 at 15:42
  • @motosubatsu Actually the OP was thinking of rotational inertia of the wheels. This is particularly felt when riding a bicycle, and then the (rotational) inertia of the tires makes a huge difference. – Calaf Jun 17 at 23:40
  • @Calaf My apologies - I had misinterpreted your initial question. Yes reducing the weight will reduce the rotational inertia - the overall impact remains very small though. The difference is only 2-5lbs per tire depending on size. – motosubatsu Jun 18 at 8:31

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