What is the diameter difference between 295/35R20 and 305/30R21?

295/35R20 has an overall diameter of 28.13 inches (715 mm). 305/30R21 is 28.2 inches (716 mm). That's a 0.07 inch (2 mm) difference — 305/30R21 is taller.

Will 305/30R21 cause speedometer error on a car fitted with 295/35R20?

Swapping 295/35R20 for 305/30R21 produces a speedometer error of about 0.3%. Your speedometer will read lower than your actual speed — a 60 mph indicated reading is really 60.2 mph. Anything under 3% is usually acceptable without recalibration.

How many revolutions per mile does each size do?

295/35R20 turns 717 revs per mile. 305/30R21 turns 715 revs per mile. Taller tyres turn fewer times per mile, which lowers effective final-drive ratio.

What's the sidewall height difference?

295/35R20 has a sidewall of 103 mm (4.06 in). 305/30R21 sidewall is 92 mm (3.6 in). Lower-profile sidewalls give sharper steering response but less bump compliance.

Is 305/30R21 a safe plus-size upgrade from 295/35R20?

A plus-size upgrade is considered safe when overall diameter stays within ±3% — here the difference is 0.3%. This swap is within the typical safe range; speedometer and ABS calibration should remain close to factory.

Tire size comparison

295/35R20 vs 305/30R21

Overall diameter, sidewall, revs per mile, and speedometer error — 305/30R21 is the taller tire, with a 0.3% speedo delta when swapping.

295/35R20

28.13in

715 mm overall diameter

305/30R21

28.2in

716 mm overall diameter

Diameter Δ

0.07 in

Speedo error

0.3%

295/35R20 revs/mi

717

305/30R21 revs/mi

715

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	295/35R20	305/30R21
Section width	295 mm	305 mm
Aspect ratio	35	30
Wheel diameter	20″	21″
Sidewall height	103 mm	92 mm
Overall diameter	28.13 in (715 mm)	28.2 in (716 mm)
Circumference	88.37 in	88.61 in
Revolutions / mile	717	715

Other comparisons for these sizes

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

FAQ

Frequently asked

What is the diameter difference between 295/35R20 and 305/30R21?: 295/35R20 has an overall diameter of 28.13 inches (715 mm). 305/30R21 is 28.2 inches (716 mm). That's a 0.07 inch (2 mm) difference — 305/30R21 is taller.
Will 305/30R21 cause speedometer error on a car fitted with 295/35R20?: Swapping 295/35R20 for 305/30R21 produces a speedometer error of about 0.3%. Your speedometer will read lower than your actual speed — a 60 mph indicated reading is really 60.2 mph. Anything under 3% is usually acceptable without recalibration.
How many revolutions per mile does each size do?: 295/35R20 turns 717 revs per mile. 305/30R21 turns 715 revs per mile. Taller tyres turn fewer times per mile, which lowers effective final-drive ratio.
What's the sidewall height difference?: 295/35R20 has a sidewall of 103 mm (4.06 in). 305/30R21 sidewall is 92 mm (3.6 in). Lower-profile sidewalls give sharper steering response but less bump compliance.
Is 305/30R21 a safe plus-size upgrade from 295/35R20?: A plus-size upgrade is considered safe when overall diameter stays within ±3% — here the difference is 0.3%. This swap is within the typical safe range; speedometer and ABS calibration should remain close to factory.