Tire size comparison
255/40R19 vs 275/30R21
Overall diameter, sidewall, revs per mile, and speedometer error — 275/30R21 is the taller tire, with a 1.7% speedo delta when swapping.
255/40R19
27.03in
687 mm overall diameter
275/30R21
27.5in
698 mm overall diameter
Diameter Δ
0.46 in
Speedo error
1.7%
255/40R19 revs/mi
746
275/30R21 revs/mi
733
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 | 255/40R19 | 275/30R21 |
|---|---|---|
| Section width | 255 mm | 275 mm |
| Aspect ratio | 40 | 30 |
| Wheel diameter | 19″ | 21″ |
| Sidewall height | 102 mm | 83 mm |
| Overall diameter | 27.03 in (687 mm) | 27.5 in (698 mm) |
| Circumference | 84.92 in | 86.38 in |
| Revolutions / mile | 746 | 733 |
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FAQ
Frequently asked
- What is the diameter difference between 255/40R19 and 275/30R21?
- 255/40R19 has an overall diameter of 27.03 inches (687 mm). 275/30R21 is 27.5 inches (698 mm). That's a 0.46 inch (12 mm) difference — 275/30R21 is taller.
- Will 275/30R21 cause speedometer error on a car fitted with 255/40R19?
- Swapping 255/40R19 for 275/30R21 produces a speedometer error of about 1.7%. Your speedometer will read lower than your actual speed — a 60 mph indicated reading is really 61 mph. Anything under 3% is usually acceptable without recalibration.
- How many revolutions per mile does each size do?
- 255/40R19 turns 746 revs per mile. 275/30R21 turns 733 revs per mile. Taller tyres turn fewer times per mile, which lowers effective final-drive ratio.
- What's the sidewall height difference?
- 255/40R19 has a sidewall of 102 mm (4.02 in). 275/30R21 sidewall is 83 mm (3.25 in). Lower-profile sidewalls give sharper steering response but less bump compliance.
- Is 275/30R21 a safe plus-size upgrade from 255/40R19?
- A plus-size upgrade is considered safe when overall diameter stays within ±3% — here the difference is 1.7%. This swap is within the typical safe range; speedometer and ABS calibration should remain close to factory.
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