205/75R13 vs 185/85R13
Going from one to the other changes the overall height by +1.1 %. That is inside the 3% band, so it fits and the speedometer stays close.
Side by side
| Measure | 205/75R13 | 185/85R13 | Change |
|---|---|---|---|
| Overall height | 25.1" | 25.4" | +1.1 % |
| Sidewall | 6.05" | 6.19" | +2.3 % |
| Section width | 205 mm | 185 mm | -9.8 % |
| Circumference | 78.9" | 79.7" | +1.1 % |
| Revs per mile | 803 | 795 | -1.1 % |
| Axle height | 12.55" | 12.69" | +3.5 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 205/75R13. Fit 185/85R13 and the needle stops telling the truth.
| Needle says | You are doing | Out by |
|---|---|---|
| 30 mph | 30.3 mph | +0.3 |
| 50 mph | 50.5 mph | +0.5 |
| 70 mph | 70.8 mph | +0.8 |
| 90 mph | 91.0 mph | +1.0 |
The odometer drifts with it. Ten thousand miles on the dial are really 10,110 on the road, so it under-counts by 110. At a true 70 mph the first turns 937 times a minute, the second 927.