205/85R18 vs 195/85R18
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 | 205/85R18 | 195/85R18 | Change |
|---|---|---|---|
| Overall height | 31.7" | 31.1" | -2.1 % |
| Sidewall | 6.86" | 6.53" | -4.9 % |
| Section width | 205 mm | 195 mm | -4.9 % |
| Circumference | 99.7" | 97.6" | -2.1 % |
| Revs per mile | 636 | 650 | +2.1 % |
| Axle height | 15.86" | 15.53" | -8.5 mm |
| Approved rim width | 5.5–7.5" | 5.5–7" | overlap 5.5–7" |
Both are approved on a rim 5.5" to 7" wide. If yours is in that band, this is a tire change and nothing else.
What the speedometer will say
Calibrated for 205/85R18. Fit 195/85R18 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 9,789 on the road, so it over-counts by 211. At a true 70 mph the first turns 742 times a minute, the second 758.