205/50/R16 vs 205/55/R16

Hi guys, its weird but my MGZS first came with 205/50/R16 tyres and thats what I have always put on the car. However whenever I have searched via the car registration, it always comes up as 205/55/R16? Would it make a difference if I put one of the 55's on whilst the other 3 are '50s'? I ask as the 55s are LOTS cheaper?!...

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