255/55R21 vs 265/50R21
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 | 255/55R21 | 265/50R21 | Change |
|---|---|---|---|
| Overall height | 32.0" | 31.4" | -1.9 % |
| Sidewall | 5.52" | 5.22" | -5.5 % |
| Section width | 255 mm | 265 mm | +3.9 % |
| Circumference | 100.7" | 98.7" | -1.9 % |
| Revs per mile | 629 | 642 | +1.9 % |
| Axle height | 16.02" | 15.72" | -7.8 mm |
| Approved rim width | 7–9.5" | 7.5–9.5" | 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 255/55R21. Fit 265/50R21 and the needle stops telling the truth.
| Needle says | You are doing | Out by |
|---|---|---|
| 30 mph | 29.4 mph | -0.6 |
| 50 mph | 49.0 mph | -1.0 |
| 70 mph | 68.7 mph | -1.3 |
| 90 mph | 88.3 mph | -1.7 |
The odometer drifts with it. Ten thousand miles on the dial are really 9,810 on the road, so it over-counts by 190. At a true 70 mph the first turns 734 times a minute, the second 749.