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Realistically, it just depends on the rolling diameter of the replacement tire compared to the rest of the tires. A lot of tire and car dealers have focused on the point that different circumferences of the tire cause driveline wind-up, and therefore must cause driveline damage. Therefore, all tires must be replaced at the same time...which happens to be a great scource of income to those who follow that theory! However, that statement is taken out of context by looking at the extreme side of the story. On every car (2WD, AWD, 4WD) the tires are not exactly equally tall, anyways. Equal tire pressure on all four tires will give you a more compressed tire on the axle with more load on it. That can be temporary, like under hard braking or cornering, or it is all the time due to static weight distribution. Furthermore, people don't check their tire pressure on a regular basis, and their tires won't be equally inflated, meaning one or more tires will have a smaller rolling diameter than the other one. Cars are also not always travelling in a straight line. The only reason why vehicles have differentials is to allow for different wheels speeds as required when travelling through a curve. Therefore a difference in wheel speed is a constant occurence for the drivetrain of the vehicle. It is for that reason that manufacturers are allowing to replace only one tire as long as its difference in wear is within reason to the other three. The previously mentioned 2 to 4/32nds are quite common. Of course the idea is to replace with exactly the same tire, since tire height differs somewhat from model to model. It is also logical that they expect a tire matching the rest of the set, since different model tires also change the handling of the vehicle. So what is happening with one tire being a litlle bit taller (newer) that the other three? - on a 4WD: when mounted on the non-driven axle, just a minor amount of walking of the side and spider gears. If your differential has oil in it, then you won't hurt anythng at all, since most axle disconnect systems do exactly that, anyways. - on a AWD: part-time AWD see above. Full-time AWD: the speed difference is neglegible to the axle differential, and is reduced by half before it goes to the center differential. Small differences in speed between the axle shafts or the two drive shafts do not trigger any limited slip or locking devices, because small differences are happening constantly while the vehicle is being driven. No damage will occur. - on a 4WD in 4WD, or a part-time AWD in 4WD: those transfer case settings are only to be used in low-traction scenarios. In those conditions it is extremely common that one wheel will spin faster the the rest. Because of the conditions the vehicle is travelling in, driveline wind-up and consequent damages are not an issue at all. Spinning one tire too fast because of heavy throttle is causing the most damage in those situations. Different tire diameters play no role in low traction environments. - on an AWD in AWD mode in low traction conditions: axle or centre differential may add the wheel speed difference to the occuring amount of tire slip, and may activate center or axle locks slightly earlier than it would otherwise. Since the system is made to do this, and the vehicle is on a surface where driveline wind-up is not an issue, no damages occur. The same holds true for electronic differential brake lock...the chances that the EBL will come on are next to nil, since some speed differential is normal. In conclusion: Replacing all four tires is always the best option, since you "refresh" your ride, and you will continue to have an equal replacement interval on all four tires. If only two are an otion, put the new ones on the same axle. Replacing only one? Make sure its rolling diameter matches as closely to the rest as possible, and try to get the same tire for consistancy in sizing and handling.