GarageCalc What 285/40R21 means
285 is the section width in millimeters (11.2 in). 40 is the aspect ratio: the sidewall is 40 percent of the width, so 285 × 0.40 = 114 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 × 114 = 761 mm, or 30.0 in.
A full marking adds a load index and speed rating after the size, for example 285/40R21 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 285/40R21
Going up one wheel size, the closest match is 285/35R22 at -0.4% diameter with a 100 mm sidewall instead of 114 mm. Going down one size, 285/45R20 is +0.4% 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 285/40R21 tire?
30.0 inches (761 mm) in overall diameter: the 21-inch wheel (533 mm) plus two 114 mm sidewalls. Real tires land within about 1 to 2 percent of that depending on brand, tread depth and pressure.
What is 285/40R21 in inches?
30.0 x 11.2 R21: 30.0 inches tall, 11.2 inches wide, on a 21-inch wheel. The sidewall is 4.5 inches and the circumference is 94.2 inches, so the tire turns 673 times per mile.
What tire sizes are equivalent to 285/40R21?
Within 3 percent of the same diameter on the same 21-inch wheel: 275/40R21 (-1.1%), 295/40R21 (+1.1%), 265/45R21 (+1.4%); on a 22-inch wheel: 285/35R22 (-0.4%), 265/40R22 (+1.2%); on a 20-inch wheel: 285/45R20 (+0.4%), 275/45R20 (-0.8%). The percentage is the diameter change, which is also your speedometer error.
Can I use 295/40R21 instead of 285/40R21?
Dimensionally yes: 295/40R21 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 295 mm.