Just for curiosity, how many miles are people going before changing the OEM tires? I'm not talking about changes to enhance performance or because they were damaged but those that just wore out.
I suspect that you need to take into account some factors like average temperatures, road surface material, and driving conditions. For example, most of my driving is in hot conditions (35-40°C in the summer), on rough, winding roads. And a lot of stop/start driving due to heavy traffic. I will be lucky if I get to 40,000 km!
The answer won't be very useful really - like l/100km, tyres are subject to how the car is driven. A Bugatti Veyron's tyres "may last 10,000 miles if you're careful, though they last only 15 minutes at the car's top speed". Jeremy Clarkson on TopGear wore out a few pairs of rear tyres in one filming session.
My first vehicle with touring tires - Yokohama BlueEarth - are holding up very well. At my 30K service the report is that they have 70% of the tread left. Downside is that they seem to be very hard rubber, noisy, and lower mileage (I assume due to the wider footprint against the road.)
Just changed my Toyos on my 2016 at 45K miles (live in FL) to Michelin Defender. Pumped them immediately up to sidewall max 44psi and I can say they are much quieter and smoother than my Toyos were. Struggling to get the same MPG though, hoping that they will improve once they get broken in.
I got 20k miles on oem with 7/16ths or more tread left if I remember correctly from the 20k service report .
2012 Prius - got 105,000 miles out of the factory Goodyears. They had tread left and passed inspection with no issues. I am on track to do about the same with the Michelin's. About a 50/50 split of city/highway driving.
I am just about to replace them right now at 40k. They are down to the wear indicators and have worn more on the inside on all four tires.
Also depends on your vehicle's alignment. And running tires down to the wear bars is dangerous. The difference between a new tire and one worn down to 2/32 (legal limit in many states) in a test on a water soaked surface was an almost doubling of stopping distance! 4/32 deep tires took about 50% longer to stop than new tires. New - 195 feet 4/32 - 290 feet 2/32 - 378 feet Even more startling to me is that, on a 2/32 tire in stopping tests in the wet from 70MPH, the new tire would have stopped the car where the worn tire would only have slowed the car to 55MPH!!!! 4/32 would only have slowed the car to 45!!!! They don't cite skidpad tests but the same issues that affect stopping distance would affect your staying on the road as you round the curve in the very wet. The tire needs the depth in order to evacuate the water beneath the tread and get the tire "rubber" into contact with the pavement. If it rides up on the water, it is like driving on ball bearings. I recall driving behind a car once and seeing it hit a puddle just as it came to a bridge and then going off like a pinball bouncing off the sides of the bridge. When I stopped, sure enough balding tires. Depth does matter in the wet. In the dry, depth is not nearly as important as long as the depth is consistent. But even in dry climates, it is that occasional rain that will get you.
I think the tire manufacturers are working on this. Michelin wants there to be a standardized test for tire performance at the end of its wear life so they must think this is a competitive advantage for their tires.
Here it's 1.6 mm minimum. It needs changing before the tread has worn down to any of the tyre wear indicators, no matter how much tread is on the rest of the tyre. If they've worn unevenly, it would be best to have a full wheel alignment. Unfortunately, with tyres rotated, we don't get an indication which axle is wearing tyres (though could be both front and rear).
