What is the diameter difference between 285/35R20 and 305/35R20?

285/35R20 has an overall diameter of 27.85 inches (708 mm). 305/35R20 is 28.41 inches (722 mm). That's a 0.55 inch (14 mm) difference — 305/35R20 is taller.

Will 305/35R20 cause speedometer error on a car fitted with 285/35R20?

Swapping 285/35R20 for 305/35R20 produces a speedometer error of about 2%. Your speedometer will read lower than your actual speed — a 60 mph indicated reading is really 61.2 mph. Anything under 3% is usually acceptable without recalibration.

How many revolutions per mile does each size do?

285/35R20 turns 724 revs per mile. 305/35R20 turns 710 revs per mile. Taller tyres turn fewer times per mile, which lowers effective final-drive ratio.

What's the sidewall height difference?

285/35R20 has a sidewall of 100 mm (3.93 in). 305/35R20 sidewall is 107 mm (4.2 in). Lower-profile sidewalls give sharper steering response but less bump compliance.

Is 305/35R20 a safe plus-size upgrade from 285/35R20?

A plus-size upgrade is considered safe when overall diameter stays within ±3% — here the difference is 2%. This swap is within the typical safe range; speedometer and ABS calibration should remain close to factory.

Tire size comparison

285/35R20 vs 305/35R20

Overall diameter, sidewall, revs per mile, and speedometer error — 305/35R20 is the taller tire, with a 2% speedo delta when swapping.

285/35R20

27.85in

708 mm overall diameter

305/35R20

28.41in

722 mm overall diameter

Diameter Δ

0.55 in

Speedo error

285/35R20 revs/mi

724

305/35R20 revs/mi

710

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	285/35R20	305/35R20
Section width	285 mm	305 mm
Aspect ratio	35	35
Wheel diameter	20″	20″
Sidewall height	100 mm	107 mm
Overall diameter	27.85 in (708 mm)	28.41 in (722 mm)
Circumference	87.51 in	89.24 in
Revolutions / mile	724	710

Other comparisons for these sizes

265/35R20 vs 285/35R20 275/35R20 vs 285/35R20 285/30R20 vs 285/35R20 285/35R20 vs 295/30R21 285/35R20 vs 285/30R21 285/35R20 vs 295/35R20 285/35R20 vs 295/30R20 295/35R20 vs 305/35R20 305/30R20 vs 305/35R20 305/35R20 vs 315/30R21

FAQ

Frequently asked

What is the diameter difference between 285/35R20 and 305/35R20?: 285/35R20 has an overall diameter of 27.85 inches (708 mm). 305/35R20 is 28.41 inches (722 mm). That's a 0.55 inch (14 mm) difference — 305/35R20 is taller.
Will 305/35R20 cause speedometer error on a car fitted with 285/35R20?: Swapping 285/35R20 for 305/35R20 produces a speedometer error of about 2%. Your speedometer will read lower than your actual speed — a 60 mph indicated reading is really 61.2 mph. Anything under 3% is usually acceptable without recalibration.
How many revolutions per mile does each size do?: 285/35R20 turns 724 revs per mile. 305/35R20 turns 710 revs per mile. Taller tyres turn fewer times per mile, which lowers effective final-drive ratio.
What's the sidewall height difference?: 285/35R20 has a sidewall of 100 mm (3.93 in). 305/35R20 sidewall is 107 mm (4.2 in). Lower-profile sidewalls give sharper steering response but less bump compliance.
Is 305/35R20 a safe plus-size upgrade from 285/35R20?: A plus-size upgrade is considered safe when overall diameter stays within ±3% — here the difference is 2%. This swap is within the typical safe range; speedometer and ABS calibration should remain close to factory.