195/55R15 vs 185/55R15
Going from one to the other changes the overall height by -1.8 %. That is inside the 3% band, so it fits and the speedometer stays close.
Side by side
| Measure | 195/55R15 | 185/55R15 | Change |
|---|---|---|---|
| Overall height | 23.4" | 23.0" | -1.8 % |
| Sidewall | 4.22" | 4.01" | -5.1 % |
| Section width | 195 mm | 185 mm | -5.1 % |
| Circumference | 73.7" | 72.3" | -1.8 % |
| Revs per mile | 860 | 876 | +1.8 % |
| Axle height | 11.72" | 11.51" | -5.5 mm |
| Approved rim width | 5.5–7" | 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 195/55R15. Fit 185/55R15 and the needle stops telling the truth.
| Needle says | You are doing | Out by |
|---|---|---|
| 30 mph | 29.4 mph | -0.6 |
| 50 mph | 49.1 mph | -0.9 |
| 70 mph | 68.7 mph | -1.3 |
| 90 mph | 88.3 mph | -1.7 |
The odometer drifts with it. Ten thousand miles on the dial are really 9,815 on the road, so it over-counts by 185. At a true 70 mph the first turns 1004 times a minute, the second 1022.