What is the diameter difference between 305/35R20 and 315/30R21?

305/35R20 has an overall diameter of 28.41 inches (722 mm). 315/30R21 is 28.44 inches (722 mm). That's a 0.04 inch (1 mm) difference — 315/30R21 is taller.

Will 315/30R21 cause speedometer error on a car fitted with 305/35R20?

Swapping 305/35R20 for 315/30R21 produces a speedometer error of about 0.1%. Your speedometer will read lower than your actual speed — a 60 mph indicated reading is really 60.1 mph. Anything under 3% is usually acceptable without recalibration.

How many revolutions per mile does each size do?

305/35R20 turns 710 revs per mile. 315/30R21 turns 709 revs per mile. Taller tyres turn fewer times per mile, which lowers effective final-drive ratio.

What's the sidewall height difference?

305/35R20 has a sidewall of 107 mm (4.2 in). 315/30R21 sidewall is 95 mm (3.72 in). Lower-profile sidewalls give sharper steering response but less bump compliance.

Is 315/30R21 a safe plus-size upgrade from 305/35R20?

A plus-size upgrade is considered safe when overall diameter stays within ±3% — here the difference is 0.1%. This swap is within the typical safe range; speedometer and ABS calibration should remain close to factory.

Tire size comparison

305/35R20 vs 315/30R21

Overall diameter, sidewall, revs per mile, and speedometer error — 315/30R21 is the taller tire, with a 0.1% speedo delta when swapping.

305/35R20

28.41in

722 mm overall diameter

315/30R21

28.44in

722 mm overall diameter

Diameter Δ

0.04 in

Speedo error

0.1%

305/35R20 revs/mi

710

315/30R21 revs/mi

709

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	305/35R20	315/30R21
Section width	305 mm	315 mm
Aspect ratio	35	30
Wheel diameter	20″	21″
Sidewall height	107 mm	95 mm
Overall diameter	28.41 in (722 mm)	28.44 in (722 mm)
Circumference	89.24 in	89.35 in
Revolutions / mile	710	709

Other comparisons for these sizes

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

FAQ

Frequently asked

What is the diameter difference between 305/35R20 and 315/30R21?: 305/35R20 has an overall diameter of 28.41 inches (722 mm). 315/30R21 is 28.44 inches (722 mm). That's a 0.04 inch (1 mm) difference — 315/30R21 is taller.
Will 315/30R21 cause speedometer error on a car fitted with 305/35R20?: Swapping 305/35R20 for 315/30R21 produces a speedometer error of about 0.1%. Your speedometer will read lower than your actual speed — a 60 mph indicated reading is really 60.1 mph. Anything under 3% is usually acceptable without recalibration.
How many revolutions per mile does each size do?: 305/35R20 turns 710 revs per mile. 315/30R21 turns 709 revs per mile. Taller tyres turn fewer times per mile, which lowers effective final-drive ratio.
What's the sidewall height difference?: 305/35R20 has a sidewall of 107 mm (4.2 in). 315/30R21 sidewall is 95 mm (3.72 in). Lower-profile sidewalls give sharper steering response but less bump compliance.
Is 315/30R21 a safe plus-size upgrade from 305/35R20?: A plus-size upgrade is considered safe when overall diameter stays within ±3% — here the difference is 0.1%. This swap is within the typical safe range; speedometer and ABS calibration should remain close to factory.