195/45R18 vs 185/45R18
Going from one to the other changes the overall height by -1.4 %. That is inside the 3% band, so it fits and the speedometer stays close.
Side by side
| Measure | 195/45R18 | 185/45R18 | Change |
|---|---|---|---|
| Overall height | 24.9" | 24.6" | -1.4 % |
| Sidewall | 3.45" | 3.28" | -5.1 % |
| Section width | 195 mm | 185 mm | -5.1 % |
| Circumference | 78.3" | 77.1" | -1.4 % |
| Revs per mile | 810 | 821 | +1.4 % |
| Axle height | 12.45" | 12.28" | -4.5 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 195/45R18. Fit 185/45R18 and the needle stops telling the truth.
| Needle says | You are doing | Out by |
|---|---|---|
| 30 mph | 29.6 mph | -0.4 |
| 50 mph | 49.3 mph | -0.7 |
| 70 mph | 69.0 mph | -1.0 |
| 90 mph | 88.7 mph | -1.3 |
The odometer drifts with it. Ten thousand miles on the dial are really 9,858 on the road, so it over-counts by 142. At a true 70 mph the first turns 945 times a minute, the second 958.