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
Similar diameter does not establish interchangeability. Follow the vehicle manufacturer's approved sizes; see Michelin's tire-size guidance.
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. The 3 percent cutoff is a comparison filter, not a fitment approval. Use vehicle-approved sizes and check rim width, load and speed ratings, and clearances. 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. Actual dimensions depend on the tire model, tread depth, pressure and load; check the tire manufacturer specifications.
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?
Nominal size comparison only: 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. This does not confirm fitment. Check approved rim widths, load and speed ratings, and vehicle clearances. The nominal extra width adds about 0.20 in of clearance on each side of the tire and a rim width that suits 295 mm.