235/50R21 vs 255/45R21
Going from one to the other changes the overall height by -0.7 %. That is inside the 3% band, so it fits and the speedometer stays close.
Side by side
| Measure | 235/50R21 | 255/45R21 | Change |
|---|---|---|---|
| Overall height | 30.3" | 30.0" | -0.7 % |
| Sidewall | 4.63" | 4.52" | -2.3 % |
| Section width | 235 mm | 255 mm | +8.5 % |
| Circumference | 95.0" | 94.4" | -0.7 % |
| Revs per mile | 667 | 671 | +0.7 % |
| Axle height | 15.13" | 15.02" | -2.8 mm |
| Approved rim width | 6.5–8.5" | 7–9.5" | overlap 7–8.5" |
Both are approved on a rim 7" to 8.5" wide. If yours is in that band, this is a tire change and nothing else.
What the speedometer will say
Calibrated for 235/50R21. Fit 255/45R21 and the needle stops telling the truth.
| Needle says | You are doing | Out by |
|---|---|---|
| 30 mph | 29.8 mph | -0.2 |
| 50 mph | 49.6 mph | -0.4 |
| 70 mph | 69.5 mph | -0.5 |
| 90 mph | 89.4 mph | -0.6 |
The odometer drifts with it. Ten thousand miles on the dial are really 9,928 on the road, so it over-counts by 72. At a true 70 mph the first turns 778 times a minute, the second 783.