195/75R18 vs 185/75R18
Going from one to the other changes the overall height by -2.0 %. That is inside the 3% band, so it fits and the speedometer stays close.
Side by side
| Measure | 195/75R18 | 185/75R18 | Change |
|---|---|---|---|
| Overall height | 29.5" | 28.9" | -2.0 % |
| Sidewall | 5.76" | 5.46" | -5.1 % |
| Section width | 195 mm | 185 mm | -5.1 % |
| Circumference | 92.7" | 90.9" | -2.0 % |
| Revs per mile | 683 | 697 | +2.0 % |
| Axle height | 14.76" | 14.46" | -7.5 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 195/75R18. Fit 185/75R18 and the needle stops telling the truth.
| Needle says | You are doing | Out by |
|---|---|---|
| 30 mph | 29.4 mph | -0.6 |
| 50 mph | 49.0 mph | -1.0 |
| 70 mph | 68.6 mph | -1.4 |
| 90 mph | 88.2 mph | -1.8 |
The odometer drifts with it. Ten thousand miles on the dial are really 9,800 on the road, so it over-counts by 200. At a true 70 mph the first turns 797 times a minute, the second 813.