Just because on day to day driving it hasn't made any difference, doesn't mean that it's ok. You're changing the handling characteristics from one part of the car to another.
You could put one run flat, one winter, one space saver and one normal tyre on and (ok so the space saver will cause probs) generally, everything would be fine for the most part. Then someone swerves towards you on a bend on a dual carriageway. You have to take evasive action. You crash.
Been trying to say this to folk on the e46 forum. Just because something (in their case £30 cool overs) works when you potter round to your grandmas house, doesn't mean it will save your life in an extreme situation.
No idea about mot pass or fail, but I just wouldn't risk it. I replaced all 4 tyres for the same cost as 2x rear fun flats and sold the fronts on here. Job done!
57 Sapphire Black E91 330d M-Sport
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