GarageCalc What 245/40R21 means
245 is the section width in millimeters (9.6 in). 40 is the aspect ratio: the sidewall is 40 percent of the width, so 245 × 0.40 = 98 mm (3.9 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 × 98 = 729 mm, or 28.7 in.
A full marking adds a load index and speed rating after the size, for example 245/40R21 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 245/40R21
Going up one wheel size, the closest match is 265/35R22 at +2.0% diameter with a 93 mm sidewall instead of 98 mm. Going down one size, 245/45R20 is -0.1% with a taller 110 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 245/40R21 tire?
28.7 inches (729 mm) in overall diameter: the 21-inch wheel (533 mm) plus two 98 mm sidewalls. Real tires land within about 1 to 2 percent of that depending on brand, tread depth and pressure.
What is 245/40R21 in inches?
28.7 x 9.6 R21: 28.7 inches tall, 9.6 inches wide, on a 21-inch wheel. The sidewall is 3.9 inches and the circumference is 90.2 inches, so the tire turns 702 times per mile.
What tire sizes are equivalent to 245/40R21?
Within 3 percent of the same diameter on the same 21-inch wheel: 255/40R21 (+1.1%), 265/35R21 (-1.4%), 265/40R21 (+2.2%); on a 22-inch wheel: 265/35R22 (+2.0%), 255/30R22 (-2.4%); on a 20-inch wheel: 245/45R20 (-0.1%), 255/45R20 (+1.1%). The percentage is the diameter change, which is also your speedometer error.
Can I use 255/40R21 instead of 245/40R21?
Dimensionally yes: 255/40R21 is 1.1% taller (+8 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 255 mm.