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I have to replace one tyre on my Legacy due to sidewall damage. The tyres all have about 4 mm tread left. Are there any issues if I only replace the one tyre regarding height/ circumference differences between new tyre (8 mm tread) and the ones I have left. I think I read somewhere that on an AWD vehicle you should replace both tyres on the same axle to avoid any damage to the drivetrain. Can anyone offer guidance on this?

On any car you if you do most things to one side of an axel, tires, wheels, brakes, suspension. You should do the same to the other side of the axel.

And on awd cars you should do all 4 corners

Matt's right; circumference differences front-and-rear make the AWD system mad.

Some tire sellers have machines that they can use to wear down a new tire to match your current tires... you may want to ask around and see if you can get that done. It kind of sucks since you're wasting good tread, but if you buy four new tires you're wasting the good tread left on your other tires.

subaru says you can exchange just one if you have less than 1/4 inch of wear. I blew up a wrx re-92, so i got a used one for free off nasioc. 4 new tires is expensive!!!

If you cant replace all four at least replace two and put one on the front and one on the rear to save the center differential.

I may be being daft, but I thought the whole concept of a centre differential was to allow for different speeds on different wheels. Why should you need to change a front and rear tyre instead of across an axle?

I understand the concept of 'if you change the left disc, change the right one too' because it upsets the balance of the vehicle but don't understand why you'd particularly need to worry about front/rear balance. After all, the centre diff already copes with such things as a variable power shift front/rear and the fact that in a turn each wheel is actually rotating at a slightly different speed.

I'm genuinely ignorant on this - not trying to pick a fight or start an argument! Can anyone clarify

I'm also curious as it would seem there would be more center diff failures if this was such an issue. What if you wear the front tires out more due to understeer and are a typical neglectful American car owner when it comes to things such as rotation? I would think Subaru wouldn't have it's solid reputation if this caused center diff failures. They're not Land Rovers.

I could be wrong, all my Subarus have been fwd and my Datsuns were rwd so I have no ownership experience with AWD. Yet.

Steve

and when you have your tires set to slightly different pressures, or you sit on only the drivers side and flatten the left side unequally, or you go around a turn, or you actually take advantage of awd...

come on, fellas, torque bind is a problem when you yank on the ebrake to ebrake turn, but that is a pretty extreme force, and people do it all the time anyway. when was the last time you heard of center diff failure on a 5mt?

If you always have a speed differential between the front and rear, you're gonna wear out the viscous goo in the center differential. It won't be obvious that it has failed until you find yourself stuck because one axle is on ice and the viscous coupling isn't locking up enough.

what about the mechanical diffs in 5mts? Do you have a 4eat, vrg?

Sorry, I was talking about 5MTs' viscous couplings, like my car's. It's a different issue with automatics. The viscous coupling mechanical center LSD in manual transmission cars has the viscous goo that can get ruined.

Automatics also don't like wheel speed differences, but for a different reason. The difference in measured speed will make the TCU lock the multiplate transfer clutch (effectively locking the front and rear axles together), resulting in binding as the wheels try to turn different linear speeds.

so even the mechanical differential is viscous in a mt? I thought it was purely mechanical, and any goo would be for lubrication. This is the stuff that firms up when you pass an electric field over it, right?

The viscous coupling in a viscous LSD contains a fluid that firms up when it is heated. In the differential, that heat is provided by the friction of two output driveshafts spinning at different speeds.

4EATs don't use viscous differentials in the center. They don't use a traditional differential at all. Instead, there's a "multiplate transfer clutch" that regulates power transfer to the rear axle. The MPT clutch is controlled by hydraulic pressure which is regulated by the TCU by pulse-width modulating a solenoid valve.

Damn guys, come on! Either 5MT or 4EAT should have the front rear replacement like I mention. Transmission life and gas mileage will drop with significant differences.

The purpose of an open differential is to allow speed differences. The purpose of a LSD (or clutch pack in 4eat) is to LIMIT speed differences.

Which would you rather handle the different sized tires? If you put one new tire on the front and one new on the rear the center differential sees an EQUAL speed. If you put both on the rear it seems as if the rear is spinning slower. In the 4eat the rear spinning slower is EXACTLY what causes the center clutch to lock.

I have multiple first hand experiences with 4eat that tried to lock up continiously and wear the u-joints out. The same would happen in the 5mt, to a lesser extent of course.