215/55R21 vs 225/50R21
Going from one to the other changes the overall height by -1.5 %. That is inside the 3% band, so it fits and the speedometer stays close.
Side by side
| Measure | 215/55R21 | 225/50R21 | Change |
|---|---|---|---|
| Overall height | 30.3" | 29.9" | -1.5 % |
| Sidewall | 4.66" | 4.43" | -4.9 % |
| Section width | 215 mm | 225 mm | +4.7 % |
| Circumference | 95.2" | 93.8" | -1.5 % |
| Revs per mile | 665 | 675 | +1.5 % |
| Axle height | 15.16" | 14.93" | -5.8 mm |
| Approved rim width | 6–8" | 6–8.5" | overlap 6–8" |
Both are approved on a rim 6" to 8" wide. If yours is in that band, this is a tire change and nothing else.
What the speedometer will say
Calibrated for 215/55R21. Fit 225/50R21 and the needle stops telling the truth.
| Needle says | You are doing | Out by |
|---|---|---|
| 30 mph | 29.6 mph | -0.4 |
| 50 mph | 49.3 mph | -0.7 |
| 70 mph | 69.0 mph | -1.0 |
| 90 mph | 88.7 mph | -1.3 |
The odometer drifts with it. Ten thousand miles on the dial are really 9,851 on the road, so it over-counts by 149. At a true 70 mph the first turns 776 times a minute, the second 788.