205/55R18 vs 215/50R18
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 | 205/55R18 | 215/50R18 | Change |
|---|---|---|---|
| Overall height | 26.9" | 26.5" | -1.5 % |
| Sidewall | 4.44" | 4.23" | -4.7 % |
| Section width | 205 mm | 215 mm | +4.9 % |
| Circumference | 84.4" | 83.1" | -1.5 % |
| Revs per mile | 750 | 762 | +1.5 % |
| Axle height | 13.44" | 13.23" | -5.3 mm |
| Approved rim width | 5.5–7.5" | 6–8" | overlap 6–7.5" |
Both are approved on a rim 6" to 7.5" wide. If yours is in that band, this is a tire change and nothing else.
What the speedometer will say
Calibrated for 205/55R18. Fit 215/50R18 and the needle stops telling the truth.
| Needle says | You are doing | Out by |
|---|---|---|
| 30 mph | 29.5 mph | -0.5 |
| 50 mph | 49.2 mph | -0.8 |
| 70 mph | 68.9 mph | -1.1 |
| 90 mph | 88.6 mph | -1.4 |
The odometer drifts with it. Ten thousand miles on the dial are really 9,846 on the road, so it over-counts by 154. At a true 70 mph the first turns 875 times a minute, the second 889.