265/50R21 vs 255/50R21
Going from one to the other changes the overall height by -1.3 %. That is inside the 3% band, so it fits and the speedometer stays close.
Side by side
| Measure | 265/50R21 | 255/50R21 | Change |
|---|---|---|---|
| Overall height | 31.4" | 31.0" | -1.3 % |
| Sidewall | 5.22" | 5.02" | -3.8 % |
| Section width | 265 mm | 255 mm | -3.8 % |
| Circumference | 98.7" | 97.5" | -1.3 % |
| Revs per mile | 642 | 650 | +1.3 % |
| Axle height | 15.72" | 15.52" | -5.0 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 255/50R21 and the needle stops telling the truth.
| Needle says | You are doing | Out by |
|---|---|---|
| 30 mph | 29.6 mph | -0.4 |
| 50 mph | 49.4 mph | -0.6 |
| 70 mph | 69.1 mph | -0.9 |
| 90 mph | 88.9 mph | -1.1 |
The odometer drifts with it. Ten thousand miles on the dial are really 9,875 on the road, so it over-counts by 125. At a true 70 mph the first turns 749 times a minute, the second 758.