215/50R18 vs 205/55R18
Going from one to the other changes the overall height by +1.6 %. That is inside the 3% band, so it fits and the speedometer stays close.
Side by side
| Measure | 215/50R18 | 205/55R18 | Change |
|---|---|---|---|
| Overall height | 26.5" | 26.9" | +1.6 % |
| Sidewall | 4.23" | 4.44" | +4.9 % |
| Section width | 215 mm | 205 mm | -4.7 % |
| Circumference | 83.1" | 84.4" | +1.6 % |
| Revs per mile | 762 | 750 | -1.6 % |
| Axle height | 13.23" | 13.44" | +5.3 mm |
| Approved rim width | 6–8" | 5.5–7.5" | 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 215/50R18. Fit 205/55R18 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 | 71.1 mph | +1.1 |
| 90 mph | 91.4 mph | +1.4 |
The odometer drifts with it. Ten thousand miles on the dial are really 10,156 on the road, so it under-counts by 156. At a true 70 mph the first turns 889 times a minute, the second 875.