185/65R18 vs 205/55R18
Going from one to the other changes the overall height by -2.1 %. That is inside the 3% band, so it fits and the speedometer stays close.
Side by side
| Measure | 185/65R18 | 205/55R18 | Change |
|---|---|---|---|
| Overall height | 27.5" | 26.9" | -2.1 % |
| Sidewall | 4.73" | 4.44" | -6.2 % |
| Section width | 185 mm | 205 mm | +10.8 % |
| Circumference | 86.3" | 84.4" | -2.1 % |
| Revs per mile | 734 | 750 | +2.1 % |
| Axle height | 13.73" | 13.44" | -7.5 mm |
| Approved rim width | 5–6.5" | 5.5–7.5" | overlap 5.5–6.5" |
Both are approved on a rim 5.5" to 6.5" wide. If yours is in that band, this is a tire change and nothing else.
What the speedometer will say
Calibrated for 185/65R18. Fit 205/55R18 and the needle stops telling the truth.
| Needle says | You are doing | Out by |
|---|---|---|
| 30 mph | 29.4 mph | -0.6 |
| 50 mph | 48.9 mph | -1.1 |
| 70 mph | 68.5 mph | -1.5 |
| 90 mph | 88.1 mph | -1.9 |
The odometer drifts with it. Ten thousand miles on the dial are really 9,785 on the road, so it over-counts by 215. At a true 70 mph the first turns 857 times a minute, the second 875.