185/40R18 vs 195/40R18
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 | 185/40R18 | 195/40R18 | Change |
|---|---|---|---|
| Overall height | 23.8" | 24.1" | +1.3 % |
| Sidewall | 2.91" | 3.07" | +5.4 % |
| Section width | 185 mm | 195 mm | +5.4 % |
| Circumference | 74.9" | 75.8" | +1.3 % |
| Revs per mile | 846 | 835 | -1.3 % |
| Axle height | 11.91" | 12.07" | +4.0 mm |
| Approved rim width | 5–6.5" | 5.5–7" | 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 185/40R18. Fit 195/40R18 and the needle stops telling the truth.
| Needle says | You are doing | Out by |
|---|---|---|
| 30 mph | 30.4 mph | +0.4 |
| 50 mph | 50.7 mph | +0.7 |
| 70 mph | 70.9 mph | +0.9 |
| 90 mph | 91.2 mph | +1.2 |
The odometer drifts with it. Ten thousand miles on the dial are really 10,132 on the road, so it under-counts by 132. At a true 70 mph the first turns 988 times a minute, the second 975.