GarageCalc What 305/40R22 means
305 is the section width in millimeters (12.0 in). 40 is the aspect ratio: the sidewall is 40 percent of the width, so 305 × 0.40 = 122 mm (4.8 in). R is radial construction, and 22 is the wheel diameter in inches (559 mm). Stack two sidewalls on the wheel and you get the overall diameter: 559 + 2 × 122 = 803 mm, or 31.6 in.
A full marking adds a load index and speed rating after the size, for example 305/40R22 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 305/40R22
No common 23-inch size stays within 3 percent of 31.6 in. Going down one size, 285/45R21 is -1.6% 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 305/40R22 tire?
31.6 inches (803 mm) in overall diameter: the 22-inch wheel (559 mm) plus two 122 mm sidewalls. Real tires land within about 1 to 2 percent of that depending on brand, tread depth and pressure.
What is 305/40R22 in inches?
31.6 x 12.0 R22: 31.6 inches tall, 12.0 inches wide, on a 22-inch wheel. The sidewall is 4.8 inches and the circumference is 99.3 inches, so the tire turns 638 times per mile.
What tire sizes are equivalent to 305/40R22?
Within 3 percent of the same diameter on the same 22-inch wheel: 285/45R22 (+1.6%), 285/40R22 (-2.0%); on a 21-inch wheel: 285/45R21 (-1.6%). The percentage is the diameter change, which is also your speedometer error.