GarageCalc What 305/35R21 means
305 is the section width in millimeters (12.0 in). 35 is the aspect ratio: the sidewall is 35 percent of the width, so 305 × 0.35 = 107 mm (4.2 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 × 107 = 747 mm, or 29.4 in.
A full marking adds a load index and speed rating after the size, for example 305/35R21 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 305/35R21
Going up one wheel size, the closest match is 285/35R22 at +1.5% diameter with a 100 mm sidewall instead of 107 mm. Going down one size, 295/40R20 is -0.4% with a taller 118 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 305/35R21 tire?
29.4 inches (747 mm) in overall diameter: the 21-inch wheel (533 mm) plus two 107 mm sidewalls. Real tires land within about 1 to 2 percent of that depending on brand, tread depth and pressure.
What is 305/35R21 in inches?
29.4 x 12.0 R21: 29.4 inches tall, 12.0 inches wide, on a 21-inch wheel. The sidewall is 4.2 inches and the circumference is 92.4 inches, so the tire turns 686 times per mile.
What tire sizes are equivalent to 305/35R21?
Within 3 percent of the same diameter on the same 21-inch wheel: 295/35R21 (-0.9%), 285/35R21 (-1.9%), 285/40R21 (+1.9%); on a 22-inch wheel: 285/35R22 (+1.5%); on a 20-inch wheel: 295/40R20 (-0.4%), 285/40R20 (-1.5%). The percentage is the diameter change, which is also your speedometer error.