My wife’s car has a temporary spare tire, AKA a donut, that she’s had to use a couple of times. The Subaru manual warns that you shouldn’t drive over 50 mph (going from memory) and to not drive it more than 50 miles. It seems to imply that the tire will not be any good after 50 miles of usage.
Do people really replace what seems to be a perfectly good spare donut after 50 miles? I can’t see any signs of treadwear and the suckers are hard to find. My tire guy says that he can’t get them and to go to a dealer or a junkyard. I’d go to a junkyard but based on my tire, I’d have no way of knowing if the junkyard tire had been used more than its purported range.
Interesting! I never knew. And here I thought I was doing better than most by checking the pressure of the spare every so often rather than forgetting about it altogether (tip: it always needs a few pounds, and if you never check it, it most likely is under-inflated and would probably perform like a wet sponge).
The other night, when my wife barely made it home on her “going flat quickly” tire, when I checked the air in the spare, it was about 10 psi. The sidewall said that it should be 60. Fortunately, I have a compressor.
Not really, you can damage the transmission on some cars by driving too long on a donut spare. I drove 200 miles on a donut spare coming out of Brownsville on the way to Houston. Once I got into the city and using my brakes I started getting warnings that the transmission was overheating due to the difference in braking force caused by the little spare tire.
Yes, thank you. I’m planning on going through Tire Rack. I was just thinking/hoping that any old tire shop should be able to handle it without going through an intermediary.
Truth. Back when I was single, one of my housemates, who was both foolish and dead broke, got a flat tire on his Ford Taurus, put the donut spare on it, and then wound up driving on it for at least a month, and at least a thousand miles.
What finally stopped him from doing so wasn’t that the donut spare died, but that his transmission did (and he had also damaged the car’s suspension, too).
I don’t trust donuts, but I once drove one over 20 miles of jeep track and 40 miles of freeway when the left rear tire on my truck went flat in the middle of the Providence Mountains. Pretty rough track too. No problem and not a bit of visible wear when we got to the tire place in Barstow.
I have a Subaru Outback with AWD. It’s recommended that all 4 tires are replaced at once because differences in treadwear will mean the more worn ones will be rotating slightly faster than the new, which can damage the AWD.
Driving on a donut would be exacerbating this issue much worse than just differences in tread wear, so maybe that has something to do with the 50 mile recommendation?
AWD aside, I would think that the donut is gonna heat up more, especially over a long drive. But I don’t think that means you have to replace it after its been driven 50 miles. When you have the flat replaced, I’m sure the mechanic could inspect the donut for damage and if none found, use it again in the future.
Okay a tangent to this thread but that is why when I’m out on the trail, especially far from civilization, I carry two spares. Yeah, you were a ways out there in the Mojave National Preserve.
But it’s nice to know that the little donut performed well for you on the rough trail.
I saw a horrible little car, absolutely falling apart, driving on four donuts. The driver told me he lived 1 mile from where we were chatting, and he’d been using the car for a year, seldom going more than a mile or two.
Again, I think that’s more of a legal “weasel-bandaid” to deflect blame - than a technical one …
Unless you drive in a perfectly straight line, all 4 tires spin at (ever-so-slightly) different speeds anyway, as they track curves differently (easy to see w/ wet tires on dry asphalt)… ask Subaru if driving curves harms their AWD, also.
… same is true for different air-pressures … that also impacts the tire size (again, slightly) and therefore the numbers of revolutions needed for a given distance.
Rules for the temp spare on a Subaru. The temp spare is to be installed on the rear only (unless 2 identical ones are installed in the front), and a fuse inserted into the FWD fuse slot to disable AWD (making the Subaru a FWD car), if you need to change a front flat tire, first install the spare on the rear, then take the full size rear tire and install it in place of the front.
With the fuse in place it’s fine to use different tire sized (fo the rear).
It makes a difference, but it’s the amount of difference that matters, as there will always be some difference. the greater the difference the faster one will wear out the clutch packs, as they will turn faster against each other and wear faster. Quite ironically, manual transmissions however use a hydraulic fluid instead of friction clutches for AWD and are somewhat better at accommodating different wheel sizes.
It’s not required of all Subaru’s and fortunately, my wife’s isn’t one that needs it. I have read that on the interwebs but her owner’s manual makes no mention of it.