GarageCalc What 305/30R19 means
305 is the section width in millimeters (12.0 in). 30 is the aspect ratio: the sidewall is 30 percent of the width, so 305 × 0.30 = 92 mm (3.6 in). R is radial construction, and 19 is the wheel diameter in inches (483 mm). Stack two sidewalls on the wheel and you get the overall diameter: 483 + 2 × 92 = 666 mm, or 26.2 in.
A full marking adds a load index and speed rating after the size, for example 305/30R19 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 305/30R19
Going up one wheel size, the closest match is 285/30R20 at +2.0% diameter with a 86 mm sidewall instead of 92 mm. Going down one size, 295/35R18 is -0.3% with a taller 103 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 305/30R19 tire?
26.2 inches (666 mm) in overall diameter: the 19-inch wheel (483 mm) plus two 92 mm sidewalls. Real tires land within about 1 to 2 percent of that depending on brand, tread depth and pressure.
What is 305/30R19 in inches?
26.2 x 12.0 R19: 26.2 inches tall, 12.0 inches wide, on a 19-inch wheel. The sidewall is 3.6 inches and the circumference is 82.3 inches, so the tire turns 770 times per mile.
What tire sizes are equivalent to 305/30R19?
Within 3 percent of the same diameter on the same 19-inch wheel: 295/30R19 (-0.9%), 285/30R19 (-1.8%), 285/35R19 (+2.5%); on a 20-inch wheel: 285/30R20 (+2.0%), 295/30R20 (+2.9%); on an 18-inch wheel: 295/35R18 (-0.3%), 305/35R18 (+0.8%). The percentage is the diameter change, which is also your speedometer error.