As we don’t know the value of a standard deviation expressed as distance interpreting as percentages (even with a normal curve) adds nothing to understanding. In addition the interval between -1 and -3 is of no particular significance.
I agree with those who are saying that there will be cars with old tyres on them that are a risk but we are no nearer to knowing how many.
Wandering off topic to respond to what should be a ‘slam dunk’ suggestion
I once thought and experienced that motoring clubs were a good automotive resource. My impression of RACV circa this decade is they have moved on from any pretence of expertise and advice about motoring to fronting sales programs for an increasing number of other things - solar, insurance, security systems, home maintenance contracts, and so on. They publish fluff articles from time to time about new vehicles and holidays and nice drives and places to stop along the way 
Sans the details behind my dissatisfaction, not germane here, an independent shop, dealer service bay, or respected franchise location would be my go to’s for advice. We keep RACV road assistance for reasons similar to some staying with Telstra but each year I am more and more tempted to go with an alternative provider - that discussion would be for another topic.
Are the other state motor clubs following along in the ‘evolution’?
Optional.
The QLD - RACQ provides inspection services for a fee (ouch!) and most likely through the local RACQ service provider. For some it may be more reassuring than other choices.
Personal preference a second opinion from a different reputable tyre/specialist. Possibly looking to one of the free inspection and quote offers. The practicality of doing so if one lives outside a major city can be limiting. It’s a step I have needed to take on more than one instance in recent years. For one the initial recommendation proved reliable when we went elsewhere, but subsequently less costly by seeking an alternate quote. In another the initial recommendation was far from reliable - arguably intending to gain business where the work was not required. A long story.
Note many of the big name tyre and mechanical services are fronted by franchises. YMMV
This is often an issue when asking a person who is selling the solution to assess a problem. If you get your car serviced by someone that doesn’t sell tyres, they might be more honest.
So did I until an RACV battery installer tried to attach the negative lead to the positive terminal.
By evidence or reasoning?
Is there any difference in profit motive between a dealership service department or big name tyre service outlet?
Equally they also rely on reputation, by product brand and quality of their service outcomes.
My comment relates to the topic of getting tyres checked.
A place that sell tyres can inflate their profit by recommending new tyres when you don’t need them. A business that doesn’t sell tyres has no incentive to do so, they can just give you an honest appraisal.
Over the years, this is what we have experienced from our independent mechanics in Brisbane and Tasmania. As part of their service report, they provide information about condition of tyres. The mechanic in Tassie even gives a percentage of tread left and details on visual tyre condition, the later is important as one can’t readily see the inside walls of the tyre and if they are damaged. A good mechanic should do much if they value their customers and their driving safety.
Understood. Assuming they are reliable and don’t have a deal with a nearby tyre supplier?
My experience with Ford, Subaru and Toyota dealerships is their service centres include tyre rotation, balancing and alignment per the logbook recommendations. Either in-house or through a sub-contract service. It’s always been on the one bill.
Appreciated some prefer to choose the brand dealer servicing and others avoid them. In smaller cities and regional townships your options are more limited. In regional centres one size often meets all needs.
We are not spoilt for options. Our most local mechanical service business specialises in also selling tyres.
We’ve used a variety of independent, franchise chain, and dealership options over the previous 30 years.
Anecdotal evidence is abundant. I’ve experienced good and bad from all options.
I’ll agree with a previous post, look at your trailer or spare car tyre. Horses for courses. I had my 27 year old vehicle checked out at a big name tyre place, changed out the 4 on-the-ground tyres, but after an inspection of the spare, we decided to keep it as a spare. This was the original spare, 27 years old at least, and I have it mounted on the outside rear of the vehicle and always have a tyre cover on it. If need be, it should get me 100km or so at least to get the road tyre fixed.
After 27 years of the tyre sitting untouched on the same rim,
Typically we look at the 4 tyres when they are to be replaced and with the agreement of the tyre service centre select the best of the 4 to become the spare. There is usually one with adequate tread to be road legal, and to not risk being to old at the next likely replacement. This assumes one has a full size spare, not one of those city car only space savers or worse a luxury car puncture and inflation kit.
Does the often mentioned 10 year lifetime also apply to the tyre on the space saver spare?
Good point about actually getting the tyre fixed. I bought this 4wd “jeep” type car new 27 years ago and I figure the maximum distance away from my house we have driven it is 100 km. We live 45 km from the Adelaide CBD, and now both it and me are semi-retired, with the car used as our farm vehicle, so I feel a bit comfortable with the spare as a spare. And the tyre is full size.
And this may be a bit off topic, but how about tractor, riding lawn mower, bicycle, and other tyres? We have had the same tyres on our tractor for 18 years, still seem to work and maintain pressure (but I guess I am not driving the tractor at 100 kph with oncoming traffic, so if it fails, then a hassle, but not critical). And they normally cost a bit more than car tyres. Have had the same bicycle tyres on my (seldom used) bike for at least 30 years, and again, still look OK and hold air fine, tread good.
Another way of looking at this is to ask what you stand to gain/lose by not replacing them if in doubt…and there’s sufficient doubt that 10yo tyres are up to the task.
One might think that 10yo tyres are up to the task of say driving 300m to the shop and back each day. But they won’t stop you in time to avoid running over the person who doesn’t see you and steps out onto the road, and what of your own safety?
An outlay of $5-600 every 5 years or so is negligible compared to what one might lose in other ways.
Have been told that the rubber cures and goes hard. I have a set of tyres on 10 x 15 inch rims with tyres that were bought around 1988. My car has around 900HP and about a year ago I tried these tyres on my ute, they were on the car for about an 90 minutes and it was like driving on ice. These tyres have no cracks or splits and to look at them they look good and now they sit in the man shed bolted to the wall. I change my tyres every 5 years regardless of how they look and the car does less than 1000klm per year.
My 8 year old tyres have lots of tread left as i’ve only just hit 9,000 km as I walk a lot.
Also when looking to the other costs of ownership - car loans, rego, insurance, servicing, loss of capital (depreciation), fuel etc. Any of those costs over time will easily exceed the outlay required on tyres. Most are recurring or can be so on a monthly cycle.
As a larger one off the cost of tyres may stand out. Spread over time $5-$10 per month or use 1c-2c per kilometre.
It is easy to advise someone to spend $1,000 on new tyres every few years because they age, but how many tyre shops actually tell the customer what the manufacturing date is on their proud new purchase? Age is from manufacturing date not fitment date. Many new cars require $250+ tyres, not $100 tyres. Then there are ranges of quality…
Regarding hardness and braking performance a performance tyre getting old and brittle as compared to its new state might still provide better grip than a new long distance compound 5-10 years on, or as I previously stated it might degrade more quickly because of the compound.
There is nothing better than an expert opinion to assess a tyre’s state although drivers using general rules of thumb are common.
My evidence of the need to replace tyres after a certain age relates to an unused spare tyre that sat on my boat trailer, admittedly uncovered in the sun for seven years and then just “exploded.”
That is, it just burst outwards and was totally useless. It was a Chinese brand, “Sailun” that came with the trailer. So I have no idea if it was a cheap brand but I suspect so, The tyre was unused but received lots of sun, bolted to the side of the trailer, in my back yard.
That scared me enough to go and replace all three trailer tyres with ones of a reputable well known brand. Tyres are not expensive relative to the other big costs of motoring.
Nasty…lucky it was not being used otherwise the outcome could have been catastrophic.
It is unlikely to be the brand. UV from sunlight, as well as associated heating and cooling, will significantly reduce tyre life.
An option is to buy a good quality tyre cover for the spare to give it some protection from the harm of the sun.
Alternatively or in addition to a cover, if the trailer tyres wear out…look rotating the wheels to ensure that they are all used and wear out evenly.
Tyres have a manufacturing date. Check the tyre sidewall for the manufacturer’s date code. It will be in the form of a four-digit number, stating the month/week and year the tyre was produced. This is an example from Continental Tyres…
Yes. I must buy a cover for that spare tyre. These days, the boat does not get a lot of use and so I would not be bothered rotating the wheels because the tread will never wear out in my lifetime but a cover for that spare makes great sense.
