255/45R20 Tire Size in Inches

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

255/45R20
29.0" tall × 10.0" wide
On a 20-inch wheel · roughly 29.0x10.0R20 in flotation sizing
MeasurementInchesMillimetres
Overall diameter29.04738
Section width10.04255
Sidewall height4.52114.8
Wheel diameter20508
Circumference91.22317
Revolutions per mile695 (432 per km)

What 255/45R20 means

255 is the section width in millimetres (10.0 in). 45 is the aspect ratio: the sidewall is 45% as tall as the tire is wide, which works out to 114.8 mm or 4.5 in. R20 is a radial tire for a 20-inch wheel.

Overall diameter is the wheel plus two sidewalls: 20 + 2 × 4.52 = 29.0 inches. These are the nominal dimensions every maker builds to; a real 255/45R20 can measure a few tenths of an inch differently by model and inflation.

Closest equivalent sizes

Common sizes within the usual 3% of 255/45R20's overall diameter. Confirm load index, speed rating and rim width before swapping.

SizeDiameterWidthChangeSpeedometer at 60
255/50R19 new wheels 29.0 in 10.0 in +0.0% really 60.0 mph
255/40R21 new wheels 29.0 in 10.0 in -0.0% really 60.0 mph
235/65R17 new wheels 29.0 in 9.3 in -0.0% really 60.0 mph
255/55R18 new wheels 29.0 in 10.0 in +0.0% really 60.0 mph
255/60R17 new wheels 29.0 in 10.0 in +0.0% really 60.0 mph
255/65R16 new wheels 29.1 in 10.0 in +0.1% really 60.0 mph

Speedometer if you switch to 255/50R19

Your speedometer stays calibrated for 255/45R20. With a taller tire you travel faster than it shows.

Speedometer readsTrue speed
30 mph30.0 mph
45 mph45.0 mph
55 mph55.0 mph
65 mph65.0 mph
75 mph75.0 mph
Compare 255/45R20 with any size →

Shop 255/45R20 tires

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

See 255/45R20 tires on Amazon →

As an Amazon Associate we earn from qualifying purchases.

Frequently Asked Questions

A 255/45R20 tire is 29.0 inches (738 mm) tall overall. That is the 20-inch wheel plus two sidewalls of 4.5 inches each (45% of the 255 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 255 mm, or 10.0 inches, measured sidewall to sidewall on the recommended rim width. The tread itself is narrower than that. In flotation sizing, 255/45R20 is roughly a 29.0x10.0R20.

255/50R19 is the closest common size, at +0.0% overall diameter (29.0 in against 29.0 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 695 revolutions per mile (432 per kilometre), from its 29.0-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

Compare any sizes

The full calculator compares up to three sizes with speedometer error, clearance change and a to-scale diagram.

Open the tire size calculator

Other 20-inch sizes

235/35R20 235/45R20 235/55R20 245/35R20 245/40R20 245/45R20 245/50R20 255/35R20 255/40R20 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 305/55R20 295/55R20 295/60R20 305/50R20 315/35R20