205/40R18 vs 185/45R18
Going from one to the other changes the overall height by +0.4 %. That is inside the 3% band, so it fits and the speedometer stays close.
Side by side
| Measure | 205/40R18 | 185/45R18 | Change |
|---|---|---|---|
| Overall height | 24.5" | 24.6" | +0.4 % |
| Sidewall | 3.23" | 3.28" | +1.5 % |
| Section width | 205 mm | 185 mm | -9.8 % |
| Circumference | 76.8" | 77.1" | +0.4 % |
| Revs per mile | 825 | 821 | -0.4 % |
| Axle height | 12.23" | 12.28" | +1.3 mm |
| Approved rim width | 5.5–7.5" | 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 205/40R18. Fit 185/45R18 and the needle stops telling the truth.
| Needle says | You are doing | Out by |
|---|---|---|
| 30 mph | 30.1 mph | +0.1 |
| 50 mph | 50.2 mph | +0.2 |
| 70 mph | 70.3 mph | +0.3 |
| 90 mph | 90.4 mph | +0.4 |
The odometer drifts with it. Ten thousand miles on the dial are really 10,040 on the road, so it under-counts by 40. At a true 70 mph the first turns 962 times a minute, the second 958.