GarageCalc What 255/30R19 means
255 is the section width in millimeters (10.0 in). 30 is the aspect ratio: the sidewall is 30 percent of the width, so 255 × 0.30 = 77 mm (3.0 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 × 77 = 636 mm, or 25.0 in.
A full marking adds a load index and speed rating after the size, for example 255/30R19 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 255/30R19
Going up one wheel size, the closest match is 235/30R20 at +2.1% diameter with a 71 mm sidewall instead of 77 mm. Going down one size, 255/35R18 is +0.0% with a taller 89 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 255/30R19 tire?
25.0 inches (636 mm) in overall diameter: the 19-inch wheel (483 mm) plus two 77 mm sidewalls. Real tires land within about 1 to 2 percent of that depending on brand, tread depth and pressure.
What is 255/30R19 in inches?
25.0 x 10.0 R19: 25.0 inches tall, 10.0 inches wide, on a 19-inch wheel. The sidewall is 3.0 inches and the circumference is 78.6 inches, so the tire turns 806 times per mile.
What tire sizes are equivalent to 255/30R19?
Within 3 percent of the same diameter on the same 19-inch wheel: 265/30R19 (+0.9%), 235/35R19 (+1.8%), 275/30R19 (+1.9%); on a 20-inch wheel: 235/30R20 (+2.1%); on an 18-inch wheel: 255/35R18 (+0.0%), 245/35R18 (-1.1%). The percentage is the diameter change, which is also your speedometer error.
Can I use 265/30R19 instead of 255/30R19?
Dimensionally yes: 265/30R19 is 0.9% taller (+6 mm) and 10 mm wider, and at a true 60 mph the speedometer would read 59.4 mph. The extra width needs about 0.20 in of clearance on each side of the tire and a rim width that suits 265 mm.