GarageCalc What 255/45R21 means
255 is the section width in millimeters (10.0 in). 45 is the aspect ratio: the sidewall is 45 percent of the width, so 255 × 0.45 = 115 mm (4.5 in). R is radial construction, and 21 is the wheel diameter in inches (533 mm). Stack two sidewalls on the wheel and you get the overall diameter: 533 + 2 × 115 = 763 mm, or 30.0 in.
A full marking adds a load index and speed rating after the size, for example 255/45R21 91V. Those two codes set how much weight and speed the tire is certified for; look them up in the load index chart and the speed rating chart.
Plus and minus sizing from 255/45R21
Going up one wheel size, the closest match is 265/40R22 at +1.0% diameter with a 106 mm sidewall instead of 115 mm. Going down one size, 255/50R20 is +0.0% with a taller 128 mm sidewall for more comfort and pothole margin. Anything inside 3 percent keeps the speedometer, ABS and gearing effectively stock; the comparison calculator shows the full speedometer table for any pair.
Frequently asked questions
How tall is a 255/45R21 tire?
30.0 inches (763 mm) in overall diameter: the 21-inch wheel (533 mm) plus two 115 mm sidewalls. Real tires land within about 1 to 2 percent of that depending on brand, tread depth and pressure.
What is 255/45R21 in inches?
30.0 x 10.0 R21: 30.0 inches tall, 10.0 inches wide, on a 21-inch wheel. The sidewall is 4.5 inches and the circumference is 94.4 inches, so the tire turns 671 times per mile.
What tire sizes are equivalent to 255/45R21?
Within 3 percent of the same diameter on the same 21-inch wheel: 245/45R21 (-1.2%), 265/45R21 (+1.2%), 275/40R21 (-1.2%); on a 22-inch wheel: 265/40R22 (+1.0%), 275/40R22 (+2.1%); on a 20-inch wheel: 255/50R20 (+0.0%), 235/55R20 (+0.5%). The percentage is the diameter change, which is also your speedometer error.
Can I use 265/45R21 instead of 255/45R21?
Dimensionally yes: 265/45R21 is 1.2% taller (+9 mm) and 10 mm wider, and at a true 60 mph the speedometer would read 59.3 mph. The extra width needs about 0.20 in of clearance on each side of the tire and a rim width that suits 265 mm.