185/45R17 vs 205/40R17
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 | 185/45R17 | 205/40R17 | Change |
|---|---|---|---|
| Overall height | 23.6" | 23.5" | -0.4 % |
| Sidewall | 3.28" | 3.23" | -1.5 % |
| Section width | 185 mm | 205 mm | +10.8 % |
| Circumference | 74.0" | 73.7" | -0.4 % |
| Revs per mile | 856 | 860 | +0.4 % |
| Axle height | 11.78" | 11.73" | -1.3 mm |
| Approved rim width | 5–6.5" | 5.5–7.5" | 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 185/45R17. Fit 205/40R17 and the needle stops telling the truth.
| Needle says | You are doing | Out by |
|---|---|---|
| 30 mph | 29.9 mph | -0.1 |
| 50 mph | 49.8 mph | -0.2 |
| 70 mph | 69.7 mph | -0.3 |
| 90 mph | 89.6 mph | -0.4 |
The odometer drifts with it. Ten thousand miles on the dial are really 9,958 on the road, so it over-counts by 42. At a true 70 mph the first turns 999 times a minute, the second 1003.