235/50R21 vs 225/55R21
Going from one to the other changes the overall height by +1.6 %. That is inside the 3% band, so it fits and the speedometer stays close.
Side by side
| Measure | 235/50R21 | 225/55R21 | Change |
|---|---|---|---|
| Overall height | 30.3" | 30.7" | +1.6 % |
| Sidewall | 4.63" | 4.87" | +5.3 % |
| Section width | 235 mm | 225 mm | -4.3 % |
| Circumference | 95.0" | 96.6" | +1.6 % |
| Revs per mile | 667 | 656 | -1.6 % |
| Axle height | 15.13" | 15.37" | +6.3 mm |
| Approved rim width | 6.5–8.5" | 6–8.5" | overlap 6.5–8.5" |
Both are approved on a rim 6.5" 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 225/55R21 and the needle stops telling the truth.
| Needle says | You are doing | Out by |
|---|---|---|
| 30 mph | 30.5 mph | +0.5 |
| 50 mph | 50.8 mph | +0.8 |
| 70 mph | 71.1 mph | +1.1 |
| 90 mph | 91.5 mph | +1.5 |
The odometer drifts with it. Ten thousand miles on the dial are really 10,163 on the road, so it under-counts by 163. At a true 70 mph the first turns 778 times a minute, the second 765.