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

295/35R21 has an overall diameter of 29.13 inches (740 mm). 305/30R21 is 28.2 inches (716 mm). That's a 0.93 inch (24 mm) difference — 305/30R21 is shorter.

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

Swapping 295/35R21 for 305/30R21 produces a speedometer error of about 3.2%. Your speedometer will read higher than your actual speed — a 60 mph indicated reading is really 58.1 mph. Anything under 3% is usually acceptable without recalibration.

How many revolutions per mile does each size do?

295/35R21 turns 692 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?

295/35R21 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 295/35R21?

A plus-size upgrade is considered safe when overall diameter stays within ±3% — here the difference is 3.2%. This swap is outside the typical ±3% safe range and may require speedometer recalibration or affect ABS / traction control behaviour.

Tire size comparison

295/35R21 vs 305/30R21

Overall diameter, sidewall, revs per mile, and speedometer error — 295/35R21 is the taller tire, with a 3.2% speedo delta when swapping.

295/35R21

29.13in

740 mm overall diameter

305/30R21

28.2in

716 mm overall diameter

Diameter Δ

0.93 in

Speedo error

3.2%

295/35R21 revs/mi

692

305/30R21 revs/mi

715

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	295/35R21	305/30R21
Section width	295 mm	305 mm
Aspect ratio	35	30
Wheel diameter	21″	21″
Sidewall height	103 mm	92 mm
Overall diameter	29.13 in (740 mm)	28.2 in (716 mm)
Circumference	91.51 in	88.61 in
Revolutions / mile	692	715

Other comparisons for these sizes

295/35R20 vs 305/30R21 305/35R20 vs 305/30R21 275/35R21 vs 295/35R21 285/30R21 vs 305/30R21 295/30R21 vs 305/30R21 295/30R21 vs 295/35R21 295/35R21 vs 305/30R22 295/35R21 vs 295/30R22 305/30R21 vs 315/30R21

FAQ

Frequently asked

What is the diameter difference between 295/35R21 and 305/30R21?: 295/35R21 has an overall diameter of 29.13 inches (740 mm). 305/30R21 is 28.2 inches (716 mm). That's a 0.93 inch (24 mm) difference — 305/30R21 is shorter.
Will 305/30R21 cause speedometer error on a car fitted with 295/35R21?: Swapping 295/35R21 for 305/30R21 produces a speedometer error of about 3.2%. Your speedometer will read higher than your actual speed — a 60 mph indicated reading is really 58.1 mph. Anything under 3% is usually acceptable without recalibration.
How many revolutions per mile does each size do?: 295/35R21 turns 692 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?: 295/35R21 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 295/35R21?: A plus-size upgrade is considered safe when overall diameter stays within ±3% — here the difference is 3.2%. This swap is outside the typical ±3% safe range and may require speedometer recalibration or affect ABS / traction control behaviour.