GarageCalc What 305/35R18 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 18 is the wheel diameter in inches (457 mm). Stack two sidewalls on the wheel and you get the overall diameter: 457 + 2 × 107 = 671 mm, or 26.4 in.
A full marking adds a load index and speed rating after the size, for example 305/35R18 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 305/35R18
Going up one wheel size, the closest match is 305/30R19 at -0.8% diameter with a 92 mm sidewall instead of 107 mm. Going down one size, 285/40R17 is -1.6% with a taller 114 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 305/35R18 tire?
26.4 inches (671 mm) in overall diameter: the 18-inch wheel (457 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 305/35R18 in inches?
26.4 x 12.0 R18: 26.4 inches tall, 12.0 inches wide, on an 18-inch wheel. The sidewall is 4.2 inches and the circumference is 83.0 inches, so the tire turns 764 times per mile.
What tire sizes are equivalent to 305/35R18?
Within 3 percent of the same diameter on the same 18-inch wheel: 295/35R18 (-1.0%), 285/35R18 (-2.1%), 285/40R18 (+2.2%); on a 19-inch wheel: 305/30R19 (-0.8%), 295/30R19 (-1.7%); on a 17-inch wheel: 285/40R17 (-1.6%). The percentage is the diameter change, which is also your speedometer error.