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It's my rear right tire. It has a significant bulge in the sidewall.

Here are two pictures of the bulge (original URL):

enter image description here enter image description here

Do I need to replace it before I ever drive on it again? Should I put on the spare to go buy the new tire? Or is it fine to even go a little bit on the highway for a few days before I replace it?

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I'm wondering if questions like this deserve a tag like "possibly-fatal" –  Bob Cross Jan 31 '12 at 23:43
    
What are these pieces of wire extending from the side? –  sharptooth Feb 1 '12 at 11:38
    
sharptooth: All tires I've ever owned have those. They're actually little flimsy bits of rubber that taper off the tire. I believe their purpose is actually as a wear indicator. My current set of Goodyear's has them along the sides. They shouldn't break off (easily) if your tire is properly balanced and inflated. Otherwise they wear off, indicating a problem. –  Robbie Feb 7 '12 at 19:10
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@Robbie, those are not wear indicators, they are simply "flash" from the tire molding process, a byproduct of manufacturing with no purpose in the use of the tire. –  mac Aug 21 '13 at 15:15

3 Answers 3

Personally I wouldn't drive on that tire - if it delaminates while you're driving you're risking a blowout. You get bulges like that if the rubber basically separates from the various reinforcement plys in the tire and as the air in the tire heats up from driving, it'll slowly expand. All it takes then is something that pokes/cuts the bulge to turn your car into a three wheeler.

I'd slap on the spare tire and take this one down to your local tire place of least distrust. In the trunk, not on the car...

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Or is it fine to even go a little bit on the highway for a few days before I replace it?

No. Please don't drive on that tire.

You are seeing a large scale deformation in a portion of the tire that's not designed for point stresses like that. There's pretty clearly a thin layer of rubber standing between the marginally acceptable and explosively decompressive states.

And, just so we're clear, please don't drive for a couple of days on the temporary spare, either. It's not rated for that kind of workload.

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Unless, of course, your car is one of the few left that comes with a proper spare wheel instead of a "space-saver" thing... –  Nick C Jan 31 '12 at 8:47
    
@NickC, with a full size spare you can certainly expect it to last longer. –  Bob Cross Jan 31 '12 at 11:14
    
Explosive decompression is a little exciting too. Best avoided. I experienced it at low speed (35mph), but while turning and that was exciting enough. Highway speeds could be disasterous. My tire looked perfectly fine, but one of the tread blocks suddenly blew out (on a tire only a few months old). So, it can happen at any time on tires that look just fine too. Best to always be ready to wrestle your car to a controlled stop! –  Brian Knoblauch Jan 31 '12 at 12:44
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@BrianKnoblauch - offtopic, but had a great time with a blowout at 120mph in a Le Mans car. Managed to brake in a straight line down to 60mph but then things became very exciting. Very glad I had runoff on that track. Made me think just how scary that would be on a road!! –  Rory Alsop Jan 31 '12 at 13:40
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A tire guy once told me that sidewall blisters (such as what is shown here) are frequently caused by driving on severely underinflated tires. I believe this to be true. The tire flexes too much when under inflated, building up too much heat in the sidewall, eventually delaminating and blistering. This same heat buildup is what can cause catastrophic blowouts of underinflated tires. Moral: check your tire pressures regularly (with a pressure gauge!). –  mac Aug 21 '13 at 15:19

In some jurisdictions (including my own), driving with a tyre looking like that is illegal. There is a question about it on the driving test, and all such academic questions on the test are about things you must do, lest you be found to be in charge of a dangerous vehicle.

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Good point - it's also clearly illegal for a reason. –  Bob Cross Feb 16 '12 at 22:15

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