GarageCalc What 255/40R21 means
255 is the section width in millimeters (10.0 in). 40 is the aspect ratio: the sidewall is 40 percent of the width, so 255 × 0.40 = 102 mm (4.0 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 × 102 = 737 mm, or 29.0 in.
A full marking adds a load index and speed rating after the size, for example 255/40R21 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 255/40R21
Going up one wheel size, the closest match is 265/35R22 at +0.9% diameter with a 93 mm sidewall instead of 102 mm. Going down one size, 255/45R20 is +0.0% with a taller 115 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 255/40R21 tire?
29.0 inches (737 mm) in overall diameter: the 21-inch wheel (533 mm) plus two 102 mm sidewalls. Real tires land within about 1 to 2 percent of that depending on brand, tread depth and pressure.
What is 255/40R21 in inches?
29.0 x 10.0 R21: 29.0 inches tall, 10.0 inches wide, on a 21-inch wheel. The sidewall is 4.0 inches and the circumference is 91.2 inches, so the tire turns 695 times per mile.
What tire sizes are equivalent to 255/40R21?
Within 3 percent of the same diameter on the same 21-inch wheel: 245/40R21 (-1.1%), 265/40R21 (+1.1%), 275/35R21 (-1.6%); on a 22-inch wheel: 265/35R22 (+0.9%); on a 20-inch wheel: 255/45R20 (+0.0%), 245/45R20 (-1.2%). The percentage is the diameter change, which is also your speedometer error.
Can I use 265/40R21 instead of 255/40R21?
Dimensionally yes: 265/40R21 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 265 mm.