195/50R15 vs 185/50R15
Going from one to the other changes the overall height by -1.7 %. That is inside the 3% band, so it fits and the speedometer stays close.
Side by side
| Measure | 195/50R15 | 185/50R15 | Change |
|---|---|---|---|
| Overall height | 22.7" | 22.3" | -1.7 % |
| Sidewall | 3.84" | 3.64" | -5.1 % |
| Section width | 195 mm | 185 mm | -5.1 % |
| Circumference | 71.2" | 70.0" | -1.7 % |
| Revs per mile | 889 | 905 | +1.7 % |
| Axle height | 11.34" | 11.14" | -5.0 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 195/50R15. Fit 185/50R15 and the needle stops telling the truth.
| Needle says | You are doing | Out by |
|---|---|---|
| 30 mph | 29.5 mph | -0.5 |
| 50 mph | 49.1 mph | -0.9 |
| 70 mph | 68.8 mph | -1.2 |
| 90 mph | 88.4 mph | -1.6 |
The odometer drifts with it. Ten thousand miles on the dial are really 9,826 on the road, so it over-counts by 174. At a true 70 mph the first turns 1038 times a minute, the second 1056.