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 |
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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.
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