255/40R20 tire size breakdown:
255 = Tire width in millimeters (255 mm)
40 = Aspect ratio (sidewall height is 40% of 255 mm)
Sidewall height = 102 mm
R = Radial construction
20 = Wheel/rim diameter in inches
Dimensions
Sidewall height: 102 mm (4.0 inches)
Rim diameter: 20 inches
Overall tire diameter: ≈ 712 mm (28.0 inches)
Circumference: ≈ 2,237 mm (88.1 inches)
Common vehicle applications
This size is commonly found on performance SUVs and luxury sedans such as:
BMW X5
Audi Q7
Mercedes-Benz GLE
It provides a sporty appearance, responsive handling, and good road grip while maintaining reasonable ride comfort.