GarageCalc What 275/45R21 means
275 is the section width in millimeters (10.8 in). 45 is the aspect ratio: the sidewall is 45 percent of the width, so 275 × 0.45 = 124 mm (4.9 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 × 124 = 781 mm, or 30.7 in.
A full marking adds a load index and speed rating after the size, for example 275/45R21 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 275/45R21
Going up one wheel size, the closest match is 275/40R22 at -0.3% diameter with a 110 mm sidewall instead of 124 mm. Going down one size, 275/50R20 is +0.3% with a taller 138 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 275/45R21 tire?
30.7 inches (781 mm) in overall diameter: the 21-inch wheel (533 mm) plus two 124 mm sidewalls. Real tires land within about 1 to 2 percent of that depending on brand, tread depth and pressure.
What is 275/45R21 in inches?
30.7 x 10.8 R21: 30.7 inches tall, 10.8 inches wide, on a 21-inch wheel. The sidewall is 4.9 inches and the circumference is 96.6 inches, so the tire turns 656 times per mile.
What tire sizes are equivalent to 275/45R21?
Within 3 percent of the same diameter on the same 21-inch wheel: 255/50R21 (+1.0%), 265/45R21 (-1.2%), 285/45R21 (+1.2%); on a 22-inch wheel: 275/40R22 (-0.3%), 285/40R22 (+0.8%); on a 20-inch wheel: 275/50R20 (+0.3%), 295/45R20 (-0.9%). The percentage is the diameter change, which is also your speedometer error.
Can I use 285/45R21 instead of 275/45R21?
Dimensionally yes: 285/45R21 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 285 mm.