<p>I have never had 4wd so I’ve only ever replaced tires 2 at a time - but my uncle whom is a mechanic said i should always do 2 at a time unless they are really really close in tred. If there is a big difference in tred and you only replace one tire it will in turn ruin the tire on the other side or possibly your new tire and you could throw off your alignment. Your car won’t be even in driving - one tire will be larger then the other tire.</p>
<p>You should replace the tires in pairs, unless the other one is pretty new,differences in tread height will cause issues with a number of things, including traction control and ABS systems. </p>
<p>As far as AWD systems goes, no, you don’t have to replace all 4. One of the reasons they recommend that prob has to do with part time 4wd systems, a big problem is that potentially tires with different wear between front and rear could cause uneven torque on the center differential, but AWD systems use a fluid coupling that addresses that…</p>
<p>The important thing is generally to buy tires of the same type, not just size, the tread pattern on tires varies across manufacturers so having a michelin mated with a bridgestone might cause problems. </p>
<p>You are smart to check, to be honest dealers often recommend things you don’t specifically need, they make money selling tires and such, they will recommend for example replacing brake fluid yearly, when with modern brake fluids and systems you can do it at major brake servicing if needed. Auto centers like Sears are famous for selling you things you don’t need as well. A good service place will seperate out things you need to do for safety and such as opposed to recommended, and explain why.</p>