johnboyairey wrote:

I fitted a set of supersize 285 50 20 on some RR sport sized rims. I new the size would be slightly wrong on the speedo, so I downloaded a coupe of speedo apps on my phone, and at 70 on the binnacle, they are showing 70 on both apps.
Weird!

That's because when your running factory size tyres the speedo will read slightly higher, i.e. when you are actually doing 68.5mph the speedo will read 70mph, this is to ensure that you don't set off any speed cameras and then sue the car manufacturer. By law the speedo can read +10% / -0, in other words it's ok to say that your going faster than you actually are, but illegal to show the car going slower. So to allow for different size tyres they just have them read low to start with.

FYI a 255/60/18 is smaller than a 255/60/18 AT and that is smaller than MT, I don't know at which stage of wear they are 255/60/18, but with tread depths of 7 - 20+ mm the size is up and down  Yesterday I couldn't spell Engineer... Today I are one!
Inventor of the 'Guide-o-Matic automatic wheel alignment tool'
Former long term L322 owner, Up/Down graded to a Classic Tractor!