What is the diameter difference between 285/65R18 and 305/55R20?

285/65R18 has an overall diameter of 32.59 inches (828 mm). 305/55R20 is 33.21 inches (844 mm). That's a 0.62 inch (16 mm) difference — 305/55R20 is taller.

Will 305/55R20 cause speedometer error on a car fitted with 285/65R18?

Swapping 285/65R18 for 305/55R20 produces a speedometer error of about 1.9%. Your speedometer will read lower than your actual speed — a 60 mph indicated reading is really 61.1 mph. Anything under 3% is usually acceptable without recalibration.

How many revolutions per mile does each size do?

285/65R18 turns 619 revs per mile. 305/55R20 turns 607 revs per mile. Taller tyres turn fewer times per mile, which lowers effective final-drive ratio.

What's the sidewall height difference?

285/65R18 has a sidewall of 185 mm (7.29 in). 305/55R20 sidewall is 168 mm (6.6 in). Lower-profile sidewalls give sharper steering response but less bump compliance.

Is 305/55R20 a safe plus-size upgrade from 285/65R18?

A plus-size upgrade is considered safe when overall diameter stays within ±3% — here the difference is 1.9%. This swap is within the typical safe range; speedometer and ABS calibration should remain close to factory.

Tire size comparison

285/65R18 vs 305/55R20

Overall diameter, sidewall, revs per mile, and speedometer error — 305/55R20 is the taller tire, with a 1.9% speedo delta when swapping.

285/65R18

32.59in

828 mm overall diameter

305/55R20

33.21in

844 mm overall diameter

Diameter Δ

0.62 in

Speedo error

1.9%

285/65R18 revs/mi

619

305/55R20 revs/mi

607

Within the ±3% overall-diameter band usually considered safe for plus-sizing — speedometer and ABS calibration should stay close to factory.

Full spec

Side by side

Metric	285/65R18	305/55R20
Section width	285 mm	305 mm
Aspect ratio	65	55
Wheel diameter	18″	20″
Sidewall height	185 mm	168 mm
Overall diameter	32.59 in (828 mm)	33.21 in (844 mm)
Circumference	102.37 in	104.33 in
Revolutions / mile	619	607

Other comparisons for these sizes

275/65R18 vs 285/65R18

FAQ

Frequently asked

What is the diameter difference between 285/65R18 and 305/55R20?: 285/65R18 has an overall diameter of 32.59 inches (828 mm). 305/55R20 is 33.21 inches (844 mm). That's a 0.62 inch (16 mm) difference — 305/55R20 is taller.
Will 305/55R20 cause speedometer error on a car fitted with 285/65R18?: Swapping 285/65R18 for 305/55R20 produces a speedometer error of about 1.9%. Your speedometer will read lower than your actual speed — a 60 mph indicated reading is really 61.1 mph. Anything under 3% is usually acceptable without recalibration.
How many revolutions per mile does each size do?: 285/65R18 turns 619 revs per mile. 305/55R20 turns 607 revs per mile. Taller tyres turn fewer times per mile, which lowers effective final-drive ratio.
What's the sidewall height difference?: 285/65R18 has a sidewall of 185 mm (7.29 in). 305/55R20 sidewall is 168 mm (6.6 in). Lower-profile sidewalls give sharper steering response but less bump compliance.
Is 305/55R20 a safe plus-size upgrade from 285/65R18?: A plus-size upgrade is considered safe when overall diameter stays within ±3% — here the difference is 1.9%. This swap is within the typical safe range; speedometer and ABS calibration should remain close to factory.