195/40R18 vs 185/40R18
Going from one to the other changes the overall height by -1.3 %. That is inside the 3% band, so it fits and the speedometer stays close.
Side by side
| Measure | 195/40R18 | 185/40R18 | Change |
|---|---|---|---|
| Overall height | 24.1" | 23.8" | -1.3 % |
| Sidewall | 3.07" | 2.91" | -5.1 % |
| Section width | 195 mm | 185 mm | -5.1 % |
| Circumference | 75.8" | 74.9" | -1.3 % |
| Revs per mile | 835 | 846 | +1.3 % |
| Axle height | 12.07" | 11.91" | -4.0 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 195/40R18. Fit 185/40R18 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 | 69.1 mph | -0.9 |
| 90 mph | 88.8 mph | -1.2 |
The odometer drifts with it. Ten thousand miles on the dial are really 9,870 on the road, so it over-counts by 130. At a true 70 mph the first turns 975 times a minute, the second 988.