What is the diameter difference between 285/30R20 and 305/30R20?

285/30R20 has an overall diameter of 26.73 inches (679 mm). 305/30R20 is 27.2 inches (691 mm). That's a 0.47 inch (12 mm) difference — 305/30R20 is taller.

Will 305/30R20 cause speedometer error on a car fitted with 285/30R20?

Swapping 285/30R20 for 305/30R20 produces a speedometer error of about 1.8%. 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?

285/30R20 turns 754 revs per mile. 305/30R20 turns 741 revs per mile. Taller tyres turn fewer times per mile, which lowers effective final-drive ratio.

What's the sidewall height difference?

285/30R20 has a sidewall of 86 mm (3.37 in). 305/30R20 sidewall is 92 mm (3.6 in). Lower-profile sidewalls give sharper steering response but less bump compliance.

Is 305/30R20 a safe plus-size upgrade from 285/30R20?

A plus-size upgrade is considered safe when overall diameter stays within ±3% — here the difference is 1.8%. This swap is within the typical safe range; speedometer and ABS calibration should remain close to factory.

Tire size comparison

285/30R20 vs 305/30R20

Overall diameter, sidewall, revs per mile, and speedometer error — 305/30R20 is the taller tire, with a 1.8% speedo delta when swapping.

285/30R20

26.73in

679 mm overall diameter

305/30R20

27.2in

691 mm overall diameter

Diameter Δ

0.47 in

Speedo error

1.8%

285/30R20 revs/mi

754

305/30R20 revs/mi

741

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	285/30R20	305/30R20
Section width	285 mm	305 mm
Aspect ratio	30	30
Wheel diameter	20″	20″
Sidewall height	86 mm	92 mm
Overall diameter	26.73 in (679 mm)	27.2 in (691 mm)
Circumference	83.98 in	85.47 in
Revolutions / mile	754	741

Other comparisons for these sizes

275/35R19 vs 285/30R20 265/30R20 vs 285/30R20 275/30R20 vs 285/30R20 275/35R20 vs 285/30R20 285/30R20 vs 295/30R20 285/30R20 vs 285/35R20 295/30R20 vs 305/30R20 295/35R20 vs 305/30R20 305/30R20 vs 305/35R20

FAQ

Frequently asked

What is the diameter difference between 285/30R20 and 305/30R20?: 285/30R20 has an overall diameter of 26.73 inches (679 mm). 305/30R20 is 27.2 inches (691 mm). That's a 0.47 inch (12 mm) difference — 305/30R20 is taller.
Will 305/30R20 cause speedometer error on a car fitted with 285/30R20?: Swapping 285/30R20 for 305/30R20 produces a speedometer error of about 1.8%. 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?: 285/30R20 turns 754 revs per mile. 305/30R20 turns 741 revs per mile. Taller tyres turn fewer times per mile, which lowers effective final-drive ratio.
What's the sidewall height difference?: 285/30R20 has a sidewall of 86 mm (3.37 in). 305/30R20 sidewall is 92 mm (3.6 in). Lower-profile sidewalls give sharper steering response but less bump compliance.
Is 305/30R20 a safe plus-size upgrade from 285/30R20?: A plus-size upgrade is considered safe when overall diameter stays within ±3% — here the difference is 1.8%. This swap is within the typical safe range; speedometer and ABS calibration should remain close to factory.