Mathematically, the 245/55/18 is slightly shorter (about 1/2") than the stock 235/60/18 and will skew the speedo slightly. A 255/55/18 is virtually identical in height to stock but slightly wider. It's the tire overall height/diameter that affects the speedo, width will not affect the speedo. A taller tire will place more load on the drivetrain. Kinda like riding a 10 speed bike, your legs feel the increased load of a higher (taller) gear.