GarageCalc What 275/35R21 means
275 is the section width in millimeters (10.8 in). 35 is the aspect ratio: the sidewall is 35 percent of the width, so 275 × 0.35 = 96 mm (3.8 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 × 96 = 726 mm, or 28.6 in.
A full marking adds a load index and speed rating after the size, for example 275/35R21 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 275/35R21
Going up one wheel size, the closest match is 255/30R22 at -1.9% diameter with a 77 mm sidewall instead of 96 mm. Going down one size, 275/40R20 is +0.3% with a taller 110 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 275/35R21 tire?
28.6 inches (726 mm) in overall diameter: the 21-inch wheel (533 mm) plus two 96 mm sidewalls. Real tires land within about 1 to 2 percent of that depending on brand, tread depth and pressure.
What is 275/35R21 in inches?
28.6 x 10.8 R21: 28.6 inches tall, 10.8 inches wide, on a 21-inch wheel. The sidewall is 3.8 inches and the circumference is 89.8 inches, so the tire turns 706 times per mile.
What tire sizes are equivalent to 275/35R21?
Within 3 percent of the same diameter on the same 21-inch wheel: 265/35R21 (-1.0%), 285/35R21 (+1.0%), 255/40R21 (+1.6%); on a 22-inch wheel: 255/30R22 (-1.9%), 265/35R22 (+2.5%); on a 20-inch wheel: 275/40R20 (+0.3%), 265/40R20 (-0.8%). The percentage is the diameter change, which is also your speedometer error.
Can I use 285/35R21 instead of 275/35R21?
Dimensionally yes: 285/35R21 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 285 mm.