Tire size comparison
305/35R20 vs 305/30R21
Overall diameter, sidewall, revs per mile, and speedometer error — 305/35R20 is the taller tire, with a 0.7% speedo delta when swapping.
305/35R20
28.41in
722 mm overall diameter
305/30R21
28.2in
716 mm overall diameter
Diameter Δ
0.2 in
Speedo error
0.7%
305/35R20 revs/mi
710
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 | 305/35R20 | 305/30R21 |
|---|---|---|
| Section width | 305 mm | 305 mm |
| Aspect ratio | 35 | 30 |
| Wheel diameter | 20″ | 21″ |
| Sidewall height | 107 mm | 92 mm |
| Overall diameter | 28.41 in (722 mm) | 28.2 in (716 mm) |
| Circumference | 89.24 in | 88.61 in |
| Revolutions / mile | 710 | 715 |
Related
Other comparisons for these sizes
FAQ
Frequently asked
- What is the diameter difference between 305/35R20 and 305/30R21?
- 305/35R20 has an overall diameter of 28.41 inches (722 mm). 305/30R21 is 28.2 inches (716 mm). That's a 0.2 inch (5 mm) difference — 305/30R21 is shorter.
- Will 305/30R21 cause speedometer error on a car fitted with 305/35R20?
- Swapping 305/35R20 for 305/30R21 produces a speedometer error of about 0.7%. Your speedometer will read higher than your actual speed — a 60 mph indicated reading is really 59.6 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. 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?
- 305/35R20 has a sidewall of 107 mm (4.2 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 305/35R20?
- A plus-size upgrade is considered safe when overall diameter stays within ±3% — here the difference is 0.7%. This swap is within the typical safe range; speedometer and ABS calibration should remain close to factory.
Related tools