205/55R15 vs 185/65R15
Going from one to the other changes the overall height by +2.5 %. That is inside the 3% band, so it fits and the speedometer stays close.
Side by side
| Measure | 205/55R15 | 185/65R15 | Change |
|---|---|---|---|
| Overall height | 23.9" | 24.5" | +2.5 % |
| Sidewall | 4.44" | 4.73" | +6.7 % |
| Section width | 205 mm | 185 mm | -9.8 % |
| Circumference | 75.0" | 76.9" | +2.5 % |
| Revs per mile | 845 | 824 | -2.5 % |
| Axle height | 11.94" | 12.23" | +7.5 mm |
| Approved rim width | 5.5–7.5" | 5–6.5" | 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 205/55R15. Fit 185/65R15 and the needle stops telling the truth.
| Needle says | You are doing | Out by |
|---|---|---|
| 30 mph | 30.7 mph | +0.7 |
| 50 mph | 51.2 mph | +1.2 |
| 70 mph | 71.7 mph | +1.7 |
| 90 mph | 92.2 mph | +2.2 |
The odometer drifts with it. Ten thousand miles on the dial are really 10,247 on the road, so it under-counts by 247. At a true 70 mph the first turns 985 times a minute, the second 962.