GarageCalc What 305/70R18 means
305 is the section width in millimeters (12.0 in). 70 is the aspect ratio: the sidewall is 70 percent of the width, so 305 × 0.70 = 214 mm (8.4 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 × 214 = 884 mm, or 34.8 in.
A full marking adds a load index and speed rating after the size, for example 305/70R18 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 305/70R18
No common 19-inch size stays within 3 percent of 34.8 in. Going down one size, 315/70R17 is -1.3% with a taller 221 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 305/70R18 tire?
34.8 inches (884 mm) in overall diameter: the 18-inch wheel (457 mm) plus two 214 mm sidewalls. Real tires land within about 1 to 2 percent of that depending on brand, tread depth and pressure.
What is 305/70R18 in inches?
34.8 x 12.0 R18: 34.8 inches tall, 12.0 inches wide, on an 18-inch wheel. The sidewall is 8.4 inches and the circumference is 109.4 inches, so the tire turns 579 times per mile.
What tire sizes are equivalent to 305/70R18?
Within 3 percent of the same diameter on the same 18-inch wheel: 325/65R18 (-0.5%), 295/70R18 (-1.6%); on a 17-inch wheel: 315/70R17 (-1.3%), 285/75R17 (-2.8%). The percentage is the diameter change, which is also your speedometer error.
Can I use 325/65R18 instead of 305/70R18?
Dimensionally yes: 325/65R18 is 0.5% shorter (-5 mm) and 20 mm wider, and at a true 60 mph the speedometer would read 60.3 mph. The extra width needs about 0.39 in of clearance on each side of the tire and a rim width that suits 325 mm.