205/40R15 vs 185/40R15
Going from one to the other changes the overall height by -2.9 %. That is inside the 3% band, so it fits and the speedometer stays close.
Side by side
| Measure | 205/40R15 | 185/40R15 | Change |
|---|---|---|---|
| Overall height | 21.5" | 20.8" | -2.9 % |
| Sidewall | 3.23" | 2.91" | -9.8 % |
| Section width | 205 mm | 185 mm | -9.8 % |
| Circumference | 67.4" | 65.4" | -2.9 % |
| Revs per mile | 940 | 968 | +2.9 % |
| Axle height | 10.73" | 10.41" | -8.0 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 205/40R15. Fit 185/40R15 and the needle stops telling the truth.
| Needle says | You are doing | Out by |
|---|---|---|
| 30 mph | 29.1 mph | -0.9 |
| 50 mph | 48.5 mph | -1.5 |
| 70 mph | 67.9 mph | -2.1 |
| 90 mph | 87.4 mph | -2.6 |
The odometer drifts with it. Ten thousand miles on the dial are really 9,706 on the road, so it over-counts by 294. At a true 70 mph the first turns 1097 times a minute, the second 1130.