GarageCalc What 255/30R20 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 20 is the wheel diameter in inches (508 mm). Stack two sidewalls on the wheel and you get the overall diameter: 508 + 2 × 77 = 661 mm, or 26.0 in.
A full marking adds a load index and speed rating after the size, for example 255/30R20 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 255/30R20
No common 21-inch size stays within 3 percent of 26.0 in. Going down one size, 255/35R19 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 255/30R20 tire?
26.0 inches (661 mm) in overall diameter: the 20-inch wheel (508 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 255/30R20 in inches?
26.0 x 10.0 R20: 26.0 inches tall, 10.0 inches wide, on a 20-inch wheel. The sidewall is 3.0 inches and the circumference is 81.8 inches, so the tire turns 775 times per mile.
What tire sizes are equivalent to 255/30R20?
Within 3 percent of the same diameter on the same 20-inch wheel: 245/30R20 (-0.9%), 235/35R20 (+1.7%), 235/30R20 (-1.8%); on a 19-inch wheel: 255/35R19 (+0.0%), 245/35R19 (-1.0%). The percentage is the diameter change, which is also your speedometer error.
Can I use 275/30R20 instead of 255/30R20?
Dimensionally yes: 275/30R20 is 1.8% taller (+12 mm) and 20 mm wider, and at a true 60 mph the speedometer would read 58.9 mph. The extra width needs about 0.39 in of clearance on each side of the tire and a rim width that suits 275 mm.