265/50R21 vs 255/55R21
Going from one to the other changes the overall height by +1.9 %. That is inside the 3% band, so it fits and the speedometer stays close.
Side by side
| Measure | 265/50R21 | 255/55R21 | Change |
|---|---|---|---|
| Overall height | 31.4" | 32.0" | +1.9 % |
| Sidewall | 5.22" | 5.52" | +5.8 % |
| Section width | 265 mm | 255 mm | -3.8 % |
| Circumference | 98.7" | 100.7" | +1.9 % |
| Revs per mile | 642 | 629 | -1.9 % |
| Axle height | 15.72" | 16.02" | +7.8 mm |
| Approved rim width | 7.5–9.5" | 7–9.5" | overlap 7.5–9.5" |
Both are approved on a rim 7.5" to 9.5" wide. If yours is in that band, this is a tire change and nothing else.
What the speedometer will say
Calibrated for 265/50R21. Fit 255/55R21 and the needle stops telling the truth.
| Needle says | You are doing | Out by |
|---|---|---|
| 30 mph | 30.6 mph | +0.6 |
| 50 mph | 51.0 mph | +1.0 |
| 70 mph | 71.4 mph | +1.4 |
| 90 mph | 91.7 mph | +1.7 |
The odometer drifts with it. Ten thousand miles on the dial are really 10,194 on the road, so it under-counts by 194. At a true 70 mph the first turns 749 times a minute, the second 734.