225/45R21 vs 215/50R21
Going from one to the other changes the overall height by +1.7 %. That is inside the 3% band, so it fits and the speedometer stays close.
Side by side
| Measure | 225/45R21 | 215/50R21 | Change |
|---|---|---|---|
| Overall height | 29.0" | 29.5" | +1.7 % |
| Sidewall | 3.99" | 4.23" | +6.2 % |
| Section width | 225 mm | 215 mm | -4.4 % |
| Circumference | 91.0" | 92.6" | +1.7 % |
| Revs per mile | 696 | 684 | -1.7 % |
| Axle height | 14.49" | 14.73" | +6.3 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 225/45R21. Fit 215/50R21 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 | 71.2 mph | +1.2 |
| 90 mph | 91.5 mph | +1.5 |
The odometer drifts with it. Ten thousand miles on the dial are really 10,170 on the road, so it under-counts by 170. At a true 70 mph the first turns 812 times a minute, the second 799.