Hello I will help you with your question,
Due to Subaru's transfer case geometry the front/rear wheel speed has to be identical when the vehicle is traveling in a straight line. There are some slight exceptions and I will list them below.
The side to side wheel speed on the same end of the vehicle also has to be identical when the vehicle is traveling in a straight line or damage to the front or rear differential will occur. A differential allows the wheels on the same end of the car to travel at different speeds when you round a turn. The inside wheel covers less distance than the outside wheel so the difference is made up in the differential. It will work fine for short distances but if you drive 200 miles down the freeway and one wheel is turning faster than the other wheel that is mated to the same differential the gears in the differential will overheat burning the bearings and the differential will have to be overhauled or replaced.
The transfer case/clutch divides the engine torque front to rear and as it does so there will be some slippage which the engineers accounted for when they designed the system. If there is a mismatch in the wheel speed front to rear for an extended period it is the same effect as the differential and the transfer clutches will overheat and burn up requiring replacement.
The system is designed for all the wheel/tire combinations to have a circumfrence within 1/2 inch of each other. This allows for inflation, wear and manufacturing tolerances. So you do not need to replace all four tires when you have a flat but you do need to replace the tire with a similar size tire if it cannot be repaired. If all the tires are worn it would make sense to replace the entire set so that they are all matched.
I hope this answers your question but if it does not please fell free to ask follow up questions
The maximum difference is 1/2 inch circumfrence as posted above.
This is circumfrence so a little math to find the actual diameter would be needed to decide if one tire is worn more than the limit by measuring the diameter. I was taught at Subaru tech school to raise the vehicle and run a tape around the tread of all four tires if there is a question of size.
You can replace a damaged tire wit any brand or size as long as it physically is within 1/2 inch circumfrence of the other three. To maintain the handeling character of the vehicle it is best to use the same brand and size but in a pinch the measurement will protect the differentials and the transfer clutches.
I always inspect the tires of any vehicle I service and if there is a mismatch I will measure them to ensure that they fall within the 1/2 inch rule and if they do not I will note it on the repair order so that the service advisor can make the customer aware of the damage that can occur from continued operation with a mismatch
Please let me know if you have further questions about this
The wear bars on DOT spec tires are set at 2/32 inch so they will start to appear at that point or a little above. I would not run my tires to the point that the wear bars start to show as the handling especially in the rain is severely compromised with that little bit of tread left. 4/32 is about my limit before I am searching for the best tire deal I can find. I rotate regularly so when I need to replace tires it is a full set that I buy.
The 1/2 inch figure is what Subaru teaches in its training classes so I am not aware of any documentation as the ideal is to have the same size on all four corners but the trainers are very emphatic that should be no more than 1/2 difference if any. The curriculum is developed by Subaru so I'm sure they are not pulling that figure out of the air. The engineering team would be the source of the direction.
Let me know what I can do to help you with this