GarageCalc What 305/35R20 means
305 is the section width in millimeters (12.0 in). 35 is the aspect ratio: the sidewall is 35 percent of the width, so 305 × 0.35 = 107 mm (4.2 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 × 107 = 722 mm, or 28.4 in.
A full marking adds a load index and speed rating after the size, for example 305/35R20 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 305/35R20
Going up one wheel size, the closest match is 285/35R21 at +1.6% diameter with a 100 mm sidewall instead of 107 mm. Going down one size, 295/40R19 is -0.4% with a taller 118 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 305/35R20 tire?
28.4 inches (722 mm) in overall diameter: the 20-inch wheel (508 mm) plus two 107 mm sidewalls. Real tires land within about 1 to 2 percent of that depending on brand, tread depth and pressure.
What is 305/35R20 in inches?
28.4 x 12.0 R20: 28.4 inches tall, 12.0 inches wide, on a 20-inch wheel. The sidewall is 4.2 inches and the circumference is 89.2 inches, so the tire turns 710 times per mile.
What tire sizes are equivalent to 305/35R20?
Within 3 percent of the same diameter on the same 20-inch wheel: 295/35R20 (-1.0%), 315/35R20 (+1.0%), 285/35R20 (-1.9%); on a 21-inch wheel: 285/35R21 (+1.6%), 295/35R21 (+2.6%); on a 19-inch wheel: 295/40R19 (-0.4%), 285/40R19 (-1.5%). The percentage is the diameter change, which is also your speedometer error.
Can I use 315/35R20 instead of 305/35R20?
Dimensionally yes: 315/35R20 is 1.0% taller (+7 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 315 mm.