185/40R17 vs 205/40R17
Going from one to the other changes the overall height by +2.8 %. That is inside the 3% band, so it fits and the speedometer stays close.
Side by side
| Measure | 185/40R17 | 205/40R17 | Change |
|---|---|---|---|
| Overall height | 22.8" | 23.5" | +2.8 % |
| Sidewall | 2.91" | 3.23" | +10.8 % |
| Section width | 185 mm | 205 mm | +10.8 % |
| Circumference | 71.7" | 73.7" | +2.8 % |
| Revs per mile | 884 | 860 | -2.8 % |
| Axle height | 11.41" | 11.73" | +8.0 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 185/40R17. Fit 205/40R17 and the needle stops telling the truth.
| Needle says | You are doing | Out by |
|---|---|---|
| 30 mph | 30.8 mph | +0.8 |
| 50 mph | 51.4 mph | +1.4 |
| 70 mph | 71.9 mph | +1.9 |
| 90 mph | 92.5 mph | +2.5 |
The odometer drifts with it. Ten thousand miles on the dial are really 10,276 on the road, so it under-counts by 276. At a true 70 mph the first turns 1031 times a minute, the second 1003.