Its the second car does 2k miles in a yr - just local drive.
WHat is the thread measure to change tires (legal)?
Anything else?
There is tread depth (legal), and there’s also tire age (not legal, but important). Both are important.
Please tell us what car you’re talking about. Year, make, model, and any options.
How old are the tires now?
The legal tread depth is 2/32 of an inch in most places (down to the indicator built into the tire). However, if you wait until the very end, you’re at some risk of hydroplaning on wet roads, and you definitely don’t want to use such tires in snow.
I replace mine at 4/32 in summer and 6/32 in winter.
Its driven in Northern CA.
No snow
Generally I would say that six years is the maximum for tyres. After that they become un-reliable. However if you live In an area where the ozone is low and if your car is usually not in the sun, they may be OK for a few more years. It's up to you and it is difficult to tell just by looking if they are good, but if there are cracks in the sidewall you know they are bad. I would be more worried if I drove on the freeway.Get a penny and find the shallowest tread on each tire. Stick the penny in upside-down. If any of Lincoln’s head is covered, you are still OK. The closer you are to seeing all of Abe’s head, the sooner you will need to replace the tires.
Would 4/32 or 8yrs is a good time to change tires?
Yes. Rubber degrades over time. The shallower the tread the more likely it is to not stop as quickly and to experience hyrdoplaning. Any decent tire store can assess what you have now and help find decent replacements.
Yes …
If you can see even one thread protruding through the tread or the sidewall, then you need to immediately replace that tire–regardless of its age.
You reopen this after 14 yrs even though there are plenty of threads that have already answered this ad infinitum? “What a maroon” said in my best Bugs Bunny voice. 
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This car has the tires it came with (walmart tires?) except one - one was replaced while on a trip with Michelin on a Sunday - so no much choice but a used tire. So I have 3 tires at 6/7 thread and one with Michellin at 3/32! The car has been resting due to Covid and we shall be driving locally say 2.5k/yr. What might be a good strategy?
I searched and found a photo resembling your description - mine don’t have these:
You apparently have eight-year-old tires that are mismatched on brand and mismatched on tread depth. The only sensible strategy is to get four new tires.
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The tire’s age is the issue imo. No idea what Calif legal requirement are for tires. Calif mostly concerned about emissions, safety issues seems more an after thought. I expect however many shops would refuse to repair an 8 year old tire, and some might refuse to work on a car with 8 year old tires. Not saying 8 year old tires couldn’t be used for local around town low speed driving, but at 8 years, good idea to think about replacing them.
Calif, 2/32 illegal
Tire shops don’t touch tires older than 10yrs - considered bad.
Tires are vegan:)
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Now that we are getting close to the ten year mark, if no one else mentioned it, you can get a tire tread gauge fo $5. Up to 6/32 nds is green, 6-3 is yellow, and below 3 is red. Depending on time of year, I replace when it hits yellow.
Lets see , old tires that might not be of high quality even when new . One brand different . Thread measurement not the same . Posts a link about bicycle tires .
Just get four new tires and be done with it . Simple enough !
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1/16 (or should I say 2/32) front and 2/16 rear. Cars hydroplane and people die due to rear wheel hydroplaning, and it can happen easily when there is more tread on the front than the back.
Is this why shop puts the new tire on the rear?
As per Tire Rack:
When tires are replaced in pairs, the new tires should always be installed on the rear axle, with the partially worn tires moved to the front. Why? Because new tires with deeper treads on the rear axle will better maintain traction and resistance to hydroplaning - the circumstance that occurs when the tire’s tread cannot process enough water to maintain effective contact with the road, causing the vehicle to lose traction.
The reason the back axle is preferred relates to what happens when a vehicle starts to hydroplane. If new tires are mounted to the front axle versus the rear, the rear tires will lose traction in wet or slippery conditions before the front. This is dangerous, as the vehicle will tend to oversteer and spin out, potentially causing loss of control. If the front tires begin to hydroplane, the vehicle naturally wants to continue moving forward, which is called understeer. Understeer is much easier to navigate, as releasing the gas pedal will slow the vehicle, helping the driver maintain better control.
Ideally, tires should be replaced in complete sets. However, when tires are replaced in pairs, the new pair should always be installed on the rear axle.
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