Well guys, some older automatic AWD vehicles will need 4 new tires when one is beyond repair (given that the three remaining ones are at 50% or less of their life) - however, if one goes bad and the others are still pretty new, you may replace only the bad one.
Same thing, if 2 are worn down and the other two are still decent (70% or better) - you don't have to buy 4 new ones. If 2 are worn out and the other ones are at 50% or less you will have to buy 4 new tires.
Unfortunately there is no industry standard in what the manufacturers call "AWD". More
Most newer AWD are actually only sophisticated 2WD cars - they have "automatic AWD". They are the ones that power only one axle (thats why those tires wear out faster) and only engage the other axle when slippage on the main axle occurs. It is the cheaper solution for people who want the added stability of AWD to feel safer. Volvo is a good example
Drawback of that cheaper solution is (engineers often call it "hang on" solution because it has been hung onto an existing 2WD platform) that the consumer is faced with the high cost of tire replacement (all 4 need to be replaced, even if only 1 or 2 are bad). This is mainly true for older automatic AWD systems that use a viscous coupling to activate the other axle.
However, some of the automatic AWD systems are more robust than others (mainly the recent models) and a tire size difference does not immediately lead to an expensive repair. The Honda CRV and the Jeep Grand Cherokee (WJ) are among them. The more robust systems employ Haldex couplings, Gerodisc couplings or hydraulic pump systems (Honda) to activate the mostly dormant axle when needed.
There are some AWD vehicles where it does not matter what you do. Replace one tire, replace 2 - it does not matter. Those AWD vehicles are true AWD
, in the sense that they are full time 4WD or permanent 4WD powering all 4 wheels with about equal force. Most Subaru (except Impreza), BMW X5, Audi, some Toyota, Mercedes, etc. are in that group of true AWD. These AWD vehicles have a differential between the two permanently powered drive shafts. They are the ones that will survive a replacement tire that is slightly larger or slightly smaller (like when you have to buy a used (larger) tire in Mexico because nothing else is available).
I wrote "survive" because the older automatic AWD systems with viscous couplings will suffer expensive damage when the tires on one axle are slightly larger in diameter than on the other axle. Repair bills of around $1000 are not uncommon.
2009 hands-on 4WD "real life" training courses