Wheels and tyre sizes

[4256stephen]

Hi, I'm new to the range rover sport scene, I have purchased a 2010 RRS, Last night in the rain the car felt awful, sliding on roundabouts etc, checked the tyre pressures they were 34 front 37 rear, upon checking on google by numberplate it says my tyres should be 275-40 r20 I have 255-55 r20 are these the wrong size? I have attached a couple of pictures? Any info would be much appreciated.