GarageCalc What 315/30R20 means
315 is the section width in millimeters (12.4 in). 30 is the aspect ratio: the sidewall is 30 percent of the width, so 315 × 0.30 = 95 mm (3.7 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 × 95 = 697 mm, or 27.4 in.
A full marking adds a load index and speed rating after the size, for example 315/30R20 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 315/30R20
Similar diameter does not establish interchangeability. Follow the vehicle manufacturer's approved sizes; see Michelin's tire-size guidance.
No common 21-inch size stays within 3 percent of 27.4 in. Going down one size, 305/35R19 is -0.1% with a taller 107 mm sidewall for more comfort and pothole margin. The 3 percent cutoff is a comparison filter, not a fitment approval. Use vehicle-approved sizes and check rim width, load and speed ratings, and clearances. The comparison calculator shows the full speedometer table for any pair.
For the effect on road speed and distance, see our worked speedometer example. When maintaining this set, use the rotation pattern guide to check whether tire direction or different front and rear sizes restrict rotation.
Frequently asked questions
How tall is a 315/30R20 tire?
27.4 inches (697 mm) in overall diameter: the 20-inch wheel (508 mm) plus two 95 mm sidewalls. Actual dimensions depend on the tire model, tread depth, pressure and load; check the tire manufacturer specifications.
What is 315/30R20 in inches?
27.4 x 12.4 R20: 27.4 inches tall, 12.4 inches wide, on a 20-inch wheel. The sidewall is 3.7 inches and the circumference is 86.2 inches, so the tire turns 735 times per mile.
What tire sizes are equivalent to 315/30R20?
Within 3 percent of the same diameter on the same 20-inch wheel: 305/30R20 (-0.9%), 295/30R20 (-1.7%), 295/35R20 (+2.5%); on a 19-inch wheel: 305/35R19 (-0.1%), 295/35R19 (-1.1%). These are nominal diameter comparisons, not approved replacements. Check vehicle-approved sizes, rim width, load and speed ratings, and clearance before changing size.