305/55R20 Tire Size in Inches

Diameter, width, sidewall and the closest equivalent sizes, computed from the size code.

305/55R20
33.2" tall × 12.0" wide
On a 20-inch wheel · roughly 33.2x12.0R20 in flotation sizing
MeasurementInchesMillimetres
Overall diameter33.21844
Section width12.01305
Sidewall height6.60167.8
Wheel diameter20508
Circumference104.32650
Revolutions per mile607 (377 per km)

What 305/55R20 means

305 is the section width in millimetres (12.0 in). 55 is the aspect ratio: the sidewall is 55% as tall as the tire is wide, which works out to 167.8 mm or 6.6 in. R20 is a radial tire for a 20-inch wheel.

Overall diameter is the wheel plus two sidewalls: 20 + 2 × 6.60 = 33.2 inches. These are the nominal dimensions every maker builds to; a real 305/55R20 can measure a few tenths of an inch differently by model and inflation.

Closest equivalent sizes

Common sizes within the usual 3% of 305/55R20's overall diameter. Confirm load index, speed rating and rim width before swapping.

SizeDiameterWidthChangeSpeedometer at 60
275/70R18 new wheels 33.2 in 10.8 in -0.2% really 59.9 mph
255/85R16 new wheels 33.1 in 10.0 in -0.4% really 59.7 mph
275/60R20 33.0 in 10.8 in -0.7% really 59.6 mph
285/60R20 33.5 in 11.2 in +0.8% really 60.5 mph
285/75R16 new wheels 32.8 in 11.2 in -1.1% really 59.3 mph
305/70R16 new wheels 32.8 in 12.0 in -1.2% really 59.3 mph

Speedometer if you switch to 275/60R20

Your speedometer stays calibrated for 305/55R20. With a shorter tire you travel slower than it shows.

Speedometer readsTrue speed
30 mph29.8 mph
45 mph44.7 mph
55 mph54.6 mph
65 mph64.6 mph
75 mph74.5 mph
Compare 305/55R20 with any size →

Shop 305/55R20 tires

Check the load index and speed rating on your door-jamb placard before you buy, especially for LT tires used for towing.

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Frequently Asked Questions

A 305/55R20 tire is 33.2 inches (844 mm) tall overall. That is the 20-inch wheel plus two sidewalls of 6.6 inches each (55% of the 305 mm section width). Expect a real tire to measure within a few tenths of an inch of this, depending on the model and inflation.

The section width is 305 mm, or 12.0 inches, measured sidewall to sidewall on the recommended rim width. The tread itself is narrower than that. In flotation sizing, 305/55R20 is roughly a 33.2x12.0R20.

275/70R18 is the closest common size, at -0.2% overall diameter (33.2 in against 33.2 in). Most makers treat about 3% as the limit for a swap without speedometer or clearance problems, but always confirm load index, speed rating and wheel width before changing size.

About 607 revolutions per mile (377 per kilometre), from its 33.2-inch rolling diameter. Real-world figures run slightly higher because a loaded tire squats. This number is what your speedometer and odometer are calibrated against.

Last updated: October 2, 2026

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Other 20-inch sizes

235/35R20 235/45R20 235/55R20 245/35R20 245/40R20 245/45R20 245/50R20 255/35R20 255/40R20 255/45R20 255/50R20 255/55R20 265/40R20 265/45R20 265/50R20 275/35R20 275/40R20 275/45R20 275/50R20 275/55R20 275/60R20 275/65R20 285/45R20 285/50R20 285/55R20 285/60R20 285/65R20 295/35R20 295/45R20 295/55R20 295/60R20 305/50R20 315/35R20