GarageCalc What 255/35R21 means
255 is the section width in millimeters (10.0 in). 35 is the aspect ratio: the sidewall is 35 percent of the width, so 255 × 0.35 = 89 mm (3.5 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 × 89 = 712 mm, or 28.0 in.
A full marking adds a load index and speed rating after the size, for example 255/35R21 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 255/35R21
Going up one wheel size, the closest match is 255/30R22 at -0.0% diameter with a 77 mm sidewall instead of 89 mm. Going down one size, 255/40R20 is +0.0% with a taller 102 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 255/35R21 tire?
28.0 inches (712 mm) in overall diameter: the 21-inch wheel (533 mm) plus two 89 mm sidewalls. Real tires land within about 1 to 2 percent of that depending on brand, tread depth and pressure.
What is 255/35R21 in inches?
28.0 x 10.0 R21: 28.0 inches tall, 10.0 inches wide, on a 21-inch wheel. The sidewall is 3.5 inches and the circumference is 88.1 inches, so the tire turns 720 times per mile.
What tire sizes are equivalent to 255/35R21?
Within 3 percent of the same diameter on the same 21-inch wheel: 265/35R21 (+1.0%), 275/35R21 (+2.0%), 245/40R21 (+2.5%); on a 22-inch wheel: 255/30R22 (-0.0%), 245/30R22 (-0.9%); on a 20-inch wheel: 255/40R20 (+0.0%), 235/45R20 (+1.1%). The percentage is the diameter change, which is also your speedometer error.
Can I use 265/35R21 instead of 255/35R21?
Dimensionally yes: 265/35R21 is 1.0% taller (+7 mm) and 10 mm wider, and at a true 60 mph the speedometer would read 59.4 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.