225/50R21 vs 215/50R21
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 | 225/50R21 | 215/50R21 | Change |
|---|---|---|---|
| Overall height | 29.9" | 29.5" | -1.3 % |
| Sidewall | 4.43" | 4.23" | -4.4 % |
| Section width | 225 mm | 215 mm | -4.4 % |
| Circumference | 93.8" | 92.6" | -1.3 % |
| Revs per mile | 675 | 684 | +1.3 % |
| Axle height | 14.93" | 14.73" | -5.0 mm |
| Approved rim width | 6–8.5" | 6–8" | 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 225/50R21. Fit 215/50R21 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 | 69.1 mph | -0.9 |
| 90 mph | 88.8 mph | -1.2 |
The odometer drifts with it. Ten thousand miles on the dial are really 9,868 on the road, so it over-counts by 132. At a true 70 mph the first turns 788 times a minute, the second 799.