215/55R21 vs 215/50R21
Going from one to the other changes the overall height by -2.8 %. That is inside the 3% band, so it fits and the speedometer stays close.
Side by side
| Measure | 215/55R21 | 215/50R21 | Change |
|---|---|---|---|
| Overall height | 30.3" | 29.5" | -2.8 % |
| Sidewall | 4.66" | 4.23" | -9.1 % |
| Section width | 215 mm | 215 mm | 0.0 % |
| Circumference | 95.2" | 92.6" | -2.8 % |
| Revs per mile | 665 | 684 | +2.8 % |
| Axle height | 15.16" | 14.73" | -10.8 mm |
| Approved rim width | 6–8" | 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 215/55R21. Fit 215/50R21 and the needle stops telling the truth.
| Needle says | You are doing | Out by |
|---|---|---|
| 30 mph | 29.2 mph | -0.8 |
| 50 mph | 48.6 mph | -1.4 |
| 70 mph | 68.0 mph | -2.0 |
| 90 mph | 87.5 mph | -2.5 |
The odometer drifts with it. Ten thousand miles on the dial are really 9,721 on the road, so it over-counts by 279. At a true 70 mph the first turns 776 times a minute, the second 799.