Rob would be of benefit here, but I think the 225 is the width of the tyres in mm and the 50/55 is the ratio of height to width.
So on the back you'd be having a tyre that's 225mm wide and 123.5mm in height but on the front it would be 112.5mm in height.
Not sure how much impact 11mm would make for driving (I know how much impact it has in goal line technology though 
).
My front and back tyres are different. Front is 225/40/18 and back is 255/35/18. Only works out 0.75mm difference though.
As with everything in the world these days, it's liability. If he fits a 55 when the manual says 50 and your tyre blows, they're on the hook for cash and worst case scenario a death. If you approach every problem with "who is insuring this and who gets sued if it goes wrong?" it usually leads to the answer.
Cheers lads. So Barney you are driving with pretty much the setup I described. Yeah, I expected Rob to be all over that question to be honest.
And yeah, no issue with the lad's decision. Wasn't his fault and he has to cover his back I get it.
Anyway, Ive rebooked appt and getting 2 x 225/50 fitted at the rear.
