Depends if winter or summer. I let them go longer in the summer because in southern cal roads stay pretty dry. If winter is on the way I change tires early.
I'd go 4/32nd in the summer. But with winter rains on the way 6-8/32nd sounds reasonable. Or sooner
Tires are legally worn-out at 2/32nds. This doesn't mean, however, that tires will get adequate wet and/or winter traction with 3/32nds... it's best to err on the side of safety and change before the legal wear-out in severe conditions.
The legal minimum in the US is 2/32nds, however there are some states that have laws that require a minimum of 4/32nds, and some where the inspection requires a minimum of 4/32. (There's a subtle difference between a law and failing inspection, but it seems to have the same effect!) One of these days I'll do the research so I can actually quote which states are which.
From a techincal point of view, hydroplaning resistance and snow traction deteriorate as the tire wears. using these as a guide becomes an exercise in "how bad is bad enough". Many tire manufacturers are starting to recommend 4/32nds.
Trying to get the most yield from any purchase is one of several BITOGER'S BELIEFS (good potential for a thread?). In the case of tires though, with roads deteriorating everywhere, in addition to the baloney skins in wet weather you also expontentially increase your chances of changing a flat on the side of the interstate at a very bad time for you. With just that thin piece of rubber separating me from a good day and a truely ugly one, I err on the side of caution and change them sooner than later. Trying to wear tires down to the cords is a FOOLS game and one that I won't play!
maine requires 3/32 for inspection and it's illegal to drive under 2/32. Also it's illegal to drive a car that fails inspection.
The subtelty is they can ticket an out of stater for tires balder than 2/32 and an in-stater for less than 3/32.
By the time my center tread is down to 3/32 the edges are usually worn a bit more anyway.
Tires are simply too important. I'm not old, but I wish I could impart to you-all the memories of how poor were the tires of thirty and forty years ago.
20,000 miles was considered good.
4/32 is a pushing it. 6/32 is more like it.
Better to learn to drive to maximize tire life. Do the research and buy the best tire.
Then learn to get at least 70,000 miles from a set of brakes . . . it is far, very far from impossible even in todays crazy driver traffic or towing heavy loads.
70,000 is what I can do (and now my wife the same), but, shoot, my old man can get 90,000 under the same conditions (and worse); and he'll get there ahead of either of us (the advantage of driving for 67-years AND always considering it a learning experience).
With the right attitude, keeping tires in great shape for many, many miles is pretty easy. And then trying to push the safety margin never arises as a question.
Living in California, where we only see snow on television and rain is very infrequent, I typically run my tires until they are 'Slicks' I of course won't drive that particular car if there is a chance of rain. I tend to be abusive with my cars(I do take my 94 Corvette to road race events)but the tires handle and brake as well if not better than new under 2/32s. I have not noticed any increased puncture rate or any negative effect in the dry. I change them when the rubber underlayer or cord starts showing through. So long as you are cognizant of weather conditions, I do not view this as a safety issue. However, I do change out my wife's tires at 2/32's. As you can imagine, with periodic rotation, I get maximum tire life.
Apart from a Ford Thunderbird '86 that had serious handling and stopping problems, and I changed all four tires on that one in 1996 or 1997, I really do not recall having to change tires on any of my rides. So, I could not have changed them often.
Right now my daily driver is a Suzuki truck, and it is on its 6th year now, and still on OEM set of tires.
Don't forget age. Tires that are 5 or more years old should be replaced. There is a date code molded into the sidewall. It is often in an oval.
Just like belts and hoses, tires dry out over time and will fail...usually at highway speed.
I run them down until I see belt. I can do that because I have 4 mounted sets. 3 are all-seasons. Nice new tires get bought in Dec and obviously are for winter, alternating on/off with the snows. I also have a "Intermediate" worn set for Spring and Fall. Eventually, a given set gets rotated down to become summer "smokers". I run them down to belts on the hot pavement without concern about wear and if it rains, I slow way down. The hot pavement June, July and August really takes a toll on tires that have decent tread. Cooler/wetter weather makes for slower wear.
Set A- Dec, Jan, Feb
Set B- Mar, Apr, May...Sept, Oct, Nov
Set C- June, July, Aug
Right now, I put on my set of intermediate/summer...Conti Extreme with 3/32nds.
This winter has been wierd, and we now have a SUV w/ new Yokohamas for deep snow.
I'm trying to skip new tires next year (probally won't happen though) I was running Potenza G-009s with 8/32nds but was wasting them this Jan, so I went back to the Contis. I also have a fresh set of Fuzion HRis on my BBS, those wheels cannot be used in winter though.
Anyway, my point is that mutiple mounted sets, even two, makes tire lifespans much more economical. FTR- Audi quattro requires 4 idential size, brand and model of tire.
Hmm...My current tires- well, let's just say I'm getting my $$ worth out of them. I'v kinda neglectd these, only had them rotated once, current rears are pretty well down to the wear-bars, & both front tires are a bit under 3/32" tread depth. Still waiting for the local Discount Tire to get some Kumho 758's in.
At 6/32, some tires are nearly new!
At 2/32, a summer tire is just getting to the best part of its life on dry roads. On wet roads you can adjust your driving to compensate.
Usually I won't get perfectly even wear out of all four tires, and when one gets down beyond the wear bars I change them all.