What is the diameter difference between 305/30R20 and 305/35R20?

305/30R20 has an overall diameter of 27.2 inches (691 mm). 305/35R20 is 28.41 inches (722 mm). That's a 1.2 inch (31 mm) difference — 305/35R20 is taller.

Will 305/35R20 cause speedometer error on a car fitted with 305/30R20?

Swapping 305/30R20 for 305/35R20 produces a speedometer error of about 4.4%. Your speedometer will read lower than your actual speed — a 60 mph indicated reading is really 62.6 mph. Anything under 3% is usually acceptable without recalibration.

How many revolutions per mile does each size do?

305/30R20 turns 741 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?

305/30R20 has a sidewall of 92 mm (3.6 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 305/30R20?

A plus-size upgrade is considered safe when overall diameter stays within ±3% — here the difference is 4.4%. This swap is outside the typical ±3% safe range and may require speedometer recalibration or affect ABS / traction control behaviour.

Tire size comparison

305/30R20 vs 305/35R20

Overall diameter, sidewall, revs per mile, and speedometer error — 305/35R20 is the taller tire, with a 4.4% speedo delta when swapping.

305/30R20

27.2in

691 mm overall diameter

305/35R20

28.41in

722 mm overall diameter

Diameter Δ

1.2 in

Speedo error

4.4%

305/30R20 revs/mi

741

305/35R20 revs/mi

710

Outside the typical ±3% safe plus-size band — you may want speedometer recalibration, and this change can affect ABS or traction control behaviour.

Full spec

Side by side

Metric	305/30R20	305/35R20
Section width	305 mm	305 mm
Aspect ratio	30	35
Wheel diameter	20″	20″
Sidewall height	92 mm	107 mm
Overall diameter	27.2 in (691 mm)	28.41 in (722 mm)
Circumference	85.47 in	89.24 in
Revolutions / mile	741	710

Other comparisons for these sizes

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

FAQ

Frequently asked

What is the diameter difference between 305/30R20 and 305/35R20?: 305/30R20 has an overall diameter of 27.2 inches (691 mm). 305/35R20 is 28.41 inches (722 mm). That's a 1.2 inch (31 mm) difference — 305/35R20 is taller.
Will 305/35R20 cause speedometer error on a car fitted with 305/30R20?: Swapping 305/30R20 for 305/35R20 produces a speedometer error of about 4.4%. Your speedometer will read lower than your actual speed — a 60 mph indicated reading is really 62.6 mph. Anything under 3% is usually acceptable without recalibration.
How many revolutions per mile does each size do?: 305/30R20 turns 741 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?: 305/30R20 has a sidewall of 92 mm (3.6 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 305/30R20?: A plus-size upgrade is considered safe when overall diameter stays within ±3% — here the difference is 4.4%. This swap is outside the typical ±3% safe range and may require speedometer recalibration or affect ABS / traction control behaviour.