195/45R18 vs 205/40R18
Going from one to the other changes the overall height by -1.8 %. That is inside the 3% band, so it fits and the speedometer stays close.
Side by side
| Measure | 195/45R18 | 205/40R18 | Change |
|---|---|---|---|
| Overall height | 24.9" | 24.5" | -1.8 % |
| Sidewall | 3.45" | 3.23" | -6.6 % |
| Section width | 195 mm | 205 mm | +5.1 % |
| Circumference | 78.3" | 76.8" | -1.8 % |
| Revs per mile | 810 | 825 | +1.8 % |
| Axle height | 12.45" | 12.23" | -5.8 mm |
| Approved rim width | 5.5–7" | 5.5–7.5" | 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 195/45R18. Fit 205/40R18 and the needle stops telling the truth.
| Needle says | You are doing | Out by |
|---|---|---|
| 30 mph | 29.5 mph | -0.5 |
| 50 mph | 49.1 mph | -0.9 |
| 70 mph | 68.7 mph | -1.3 |
| 90 mph | 88.4 mph | -1.6 |
The odometer drifts with it. Ten thousand miles on the dial are really 9,818 on the road, so it over-counts by 182. At a true 70 mph the first turns 945 times a minute, the second 962.