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By my calculations the rolling diameter of the 255/50R20 and 255/55R19 tyres is 30 inches, and the 255/55R20 is 31 inches, which is a 3.3% increase. When the speedo thinks you are going 70 mph you will now be going 72.3 mph i.e. the speedo will under read by 3.2%. I have 30 inch vs original 29 inch tyres on my Discovery 2 and the maths are pretty much the same. Most speedos over-read (none should under-read) so I suggest that you hit the cruise control at 70 mph and then use a separate sat nav or mobile with a speedo app to measure your actual speed so at least you know for the future.
You must tell your insurers of the change in tyre size.
You must tell your insurers of the change in tyre size.