GarageCalc What 275/45R20 means
275 is the section width in millimeters (10.8 in). 45 is the aspect ratio: the sidewall is 45 percent of the width, so 275 × 0.45 = 124 mm (4.9 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 × 124 = 756 mm, or 29.7 in.
A full marking adds a load index and speed rating after the size, for example 275/45R20 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 275/45R20
Going up one wheel size, the closest match is 275/40R21 at -0.3% diameter with a 110 mm sidewall instead of 124 mm. Going down one size, 275/50R19 is +0.3% with a taller 138 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 275/45R20 tire?
29.7 inches (756 mm) in overall diameter: the 20-inch wheel (508 mm) plus two 124 mm sidewalls. Real tires land within about 1 to 2 percent of that depending on brand, tread depth and pressure.
What is 275/45R20 in inches?
29.7 x 10.8 R20: 29.7 inches tall, 10.8 inches wide, on a 20-inch wheel. The sidewall is 4.9 inches and the circumference is 93.4 inches, so the tire turns 678 times per mile.
What tire sizes are equivalent to 275/45R20?
Within 3 percent of the same diameter on the same 20-inch wheel: 255/50R20 (+1.0%), 265/45R20 (-1.2%), 285/45R20 (+1.2%); on a 21-inch wheel: 275/40R21 (-0.3%), 285/40R21 (+0.8%); on a 19-inch wheel: 275/50R19 (+0.3%), 255/55R19 (+1.0%). The percentage is the diameter change, which is also your speedometer error.
Can I use 285/45R20 instead of 275/45R20?
Dimensionally yes: 285/45R20 is 1.2% taller (+9 mm) and 10 mm wider, and at a true 60 mph the speedometer would read 59.3 mph. The extra width needs about 0.20 in of clearance on each side of the tire and a rim width that suits 285 mm.