Spare Tire Question

Hello,
Would appreciate your opinions on this, please:
How "safe" is it running on the 'toy' spare tire that is pretty much standard these days for cars, to go e.g. 50 miles at no more
than 45 mph (highway) ?
The spare is of course of a smaller diameter, hence the concern.
Anything to be concerned about if no more than 45 mph and about 50 miles or so on the highway ?
What would be the reasonable safe limits for speed and mileage ?
Thanks, B.
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I wouldn't. Mind you, I have been passed by people driving 75 mph on the doughnut, but I wouldn't do it.
If you look at the doughnut, you will find it is probably rated for 35 mph maximum. It's only there to get you to a place where you can get your flat tire repaired or replaced.

Personally, the thing gives me the willies. Go to a junkyard, get a used normal-sized rim. Take it to a tire store, get a gently used tire and have them put it on. If you pay $50 for the combination, I'll be surprised. A real spare is not only safer than the doughnut but more convenient. --scott
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"C'est un Nagra. C'est suisse, et tres, tres precis."

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Scott Dorsey wrote:

The problem with a full sized spare is usually storage room. More than a few vehicles don't have the room.
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Steve W.
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Bob wrote:

Not a problem. I see folks who drive on them for FAR more than 50 miles!

Not a big deal as long as you don't have full time 4X4 or AWD. Then I would be VERY careful and only drive to the closest garage to get the tire repaired/replaced (even a used tire of the correct size would be better than the donut in this case)

No but be aware that those spares are supposed to be a limited use item. The "expected" use is to put it on to drive to the shop. Get tire repaired/replaced AND replace the spare. I usually check them good and if they had proper inflation and the tread is still ok put them back in.

Reasonable would be the sidewall statement on the spare. BUT I have seen people driving at highway speeds for FAR more than 50 miles and not have a problem.

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First, think about the conditions...A smooth concrete or blacktop road or a "trail". Is it raining or snowing?
Second, is the doughnut properly inflated? My brand spanking new one loses a little pressure over a period of months. Shouldn't but does. If yours is not properly inflated, you may have a rather dangerous situation.
Last, these doughnuts are made to let you limp into a place where you can get a proper repair. In this part of the country you normally dont have to drive more than 15-30 miles to get as near civilization are yor are going to get. I wouldnt drive any further than I had to to get it changed.
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I've had two problems with properly inflated spares.
Twice I've had a trunk stored (full sized) spare tire explode because of high temperatures in the trunk. (When the outside temperature is above 110F the trunk can be 150F or more causing excess tire pressure.) I solved this by leaving the tire under-inflated and carrying a 12V pump to use if the spare is ever needed. This probably wouldn't be a problem unless you live in a climate such as I do in Phoenix.
My latest problem is with my Town and Country. The spare (doughnut) is mounted under the car and between the front seats. You have to use a wrench inside the car to lower the tire (and tire case) to the ground and then slide it out from under the car to check the air pressure. A pain. So same solution, I just carry a 12V pump and air up the tire just once a year or so.
And it's not a bad idea to always carry a pump anyway. It's saved me having to even change the tire when it was a slow leak and I could air it up enough to drive a few miles to the repair shop (or home)...
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If it exploded there was something wrong with the tire or you are putting it in place with little or no air in it and then pumping it up and forcing the sidewall against something sharp that cuts it. The tires on the car get far hotter then the one sitting in the trunk and they don't explode. I"ve lived in Phx my whole life and have never had a tire explode in the trunk, even really really old tires pumped up to 40 psi cold.
wrote:

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Maybe, but I think it's unlikely that I had 2 bad tires in a row. The first spare tire was brand new and came with my new 95 Taurus. The second was a used tire bought to replace it. Never had a third but by then I had started leaving the spare under-inflated.

Nope.

Tires sitting on a parked car would be at or near the local air/surface temperature. Rolling tires would also be air cooled as well as road cooled. Neither gets as hot as the inside of a trunk (or passengers compartment) on a hot AZ summer day.

Of course overheated tires explode, it's called a blowout. But in most cases rolling tire heat is caused by tire flexing due to under-inflation.

And we're supposed to pay attention to someone who lives in a hostile tire climate and touts using really really old over-inflated spare tires??
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On Mon, 22 Nov 2010 14:30:08 -0700, AJL wrote:

You need to look for another reason.
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On 11/22/2010 01:51 PM, Bret wrote:

of course. cuts, bad valves, etc. when a healthy tire is inflated until it blows, it's a really serious BANG.

http://www.youtube.com/watch?v=k3kq8iBkdoc

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Air increases in volume about 1/273 per degree C. A temperature change from -17.8 degrees C to 100 degrees C result in an increase in volume of approximately from 1 to 1.2 and an increase in pressure of about the same .
Cheers,
John B.
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My doughnut tire has an inflation pressure of 60 psig. In Arizona this past summer, as we passed through, the air temperature was 110F. The radiant heat in the trunk may have caused the temperature to rise much much hotter.
Now, I dont know what these are rated, but I can see that the pressure could increase a good bit. Gas law calculations will tell you how much, roughly.
I did not have an in-trunk blowout.. But if the poster said he did,then most likely he did.
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I don't doubt his tires "blew up". What I doubt is that his claim that all the perfectly good tires in trunks in hot climates are in danger of "blowing up". Only a damaged or defective stationary tire is going to blow up because the pressure went from 32 (at 80 degrees) to 39 psi (at 150 degrees) (1 psi per 10 degree rise in temperature).
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I agree, Ashton. It seems unlikely, but if he said that it did happen, I do not impune his honesty.
Had a relative here who owned a service station a few years ago. He was repairing a tire, and upon reassembly it blew off the rim, breaking his legs. Lucky he wasnt killed. I have no idea what pressure he reached (consider most service station compressor pressure capabilities) before it blew,and if there were other reasons it happened.
I did a rough gas law calculation, and if the temperature went from 80F to 180F, the pressure in my case would go from 60 psi to a little over 70psi. Not likely enough to blow up a healthy tire.
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On 11/21/2010 06:49 PM, AJL wrote:

i don't think so. i don't know if you know how much that kind of temperature will increase a tire's air pressure [hint: it's roughly one psi per 10°F, but it's not enough to explode a tire.
i've seen tires deliberately over-inflated to explode them, and when they go, they go with sufficient force to kill you if you're too close. it's literally like a small grenade that will take out your windows and significantly damage all your metalwork.

that is true. foot pumps are reliable and cheap and work even if your car's battery is dead.

--
nomina rutrum rutrum

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Yup, just made that shit up.

Damn, blown tire and dead battery at the same time. Maybe you should invest in AAA. But no with your luck your cell phone would be likely dead too...
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You have learned about the infamous jim beam.. More sound than fury.
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On 11/22/2010 01:55 PM, AJL wrote:

so how many windows did they blow out?

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The owners manual and tire are usually quite clear. The baby spare is safe enough to drive within the speed and distances described. Why would you think it is any different???
That said I've seen idiots driving around on baby spares that have clearly been on the car for a while.
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