GarageCalc What 275/40R21 means
275 is the section width in millimeters (10.8 in). 40 is the aspect ratio: the sidewall is 40 percent of the width, so 275 × 0.40 = 110 mm (4.3 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 × 110 = 753 mm, or 29.7 in.
A full marking adds a load index and speed rating after the size, for example 275/40R21 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 275/40R21
Going up one wheel size, the closest match is 285/35R22 at +0.7% diameter with a 100 mm sidewall instead of 110 mm. Going down one size, 275/45R20 is +0.3% with a taller 124 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 275/40R21 tire?
29.7 inches (753 mm) in overall diameter: the 21-inch wheel (533 mm) plus two 110 mm sidewalls. Real tires land within about 1 to 2 percent of that depending on brand, tread depth and pressure.
What is 275/40R21 in inches?
29.7 x 10.8 R21: 29.7 inches tall, 10.8 inches wide, on a 21-inch wheel. The sidewall is 4.3 inches and the circumference is 93.2 inches, so the tire turns 680 times per mile.
What tire sizes are equivalent to 275/40R21?
Within 3 percent of the same diameter on the same 21-inch wheel: 265/40R21 (-1.1%), 285/40R21 (+1.1%), 255/45R21 (+1.3%); on a 22-inch wheel: 285/35R22 (+0.7%), 265/35R22 (-1.2%); on a 20-inch wheel: 275/45R20 (+0.3%), 265/45R20 (-0.9%). The percentage is the diameter change, which is also your speedometer error.
Can I use 285/40R21 instead of 275/40R21?
Dimensionally yes: 285/40R21 is 1.1% taller (+8 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.