Tire Replacement

From what i've read, there should be no more than 2mm tread difference on the same axle, and no more than a 3mm thread difference between axles.

It's apparently to reduce the strain put on the diffs with individual tyres rotating at slightly different speeds.

Also from what i've read, some tyre places will shave your new wheel to within tolerances to match your existing ones.

I've always replaced tyres in pairs and never had an issue, although the last time i had to replace all 4 cos my rears were nearly done and i had a sidewall bulge in one of my 'nearly new' fronts (6mm tread left) after clobbering a pothole on the motorway.