205/80R13 vs 185/85R13
Going from one to the other changes the overall height by -2.1 %. That is inside the 3% band, so it fits and the speedometer stays close.
Side by side
| Measure | 205/80R13 | 185/85R13 | Change |
|---|---|---|---|
| Overall height | 25.9" | 25.4" | -2.1 % |
| Sidewall | 6.46" | 6.19" | -4.1 % |
| Section width | 205 mm | 185 mm | -9.8 % |
| Circumference | 81.4" | 79.7" | -2.1 % |
| Revs per mile | 778 | 795 | +2.1 % |
| Axle height | 12.96" | 12.69" | -6.8 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 205/80R13. Fit 185/85R13 and the needle stops telling the truth.
| Needle says | You are doing | Out by |
|---|---|---|
| 30 mph | 29.4 mph | -0.6 |
| 50 mph | 49.0 mph | -1.0 |
| 70 mph | 68.6 mph | -1.4 |
| 90 mph | 88.2 mph | -1.8 |
The odometer drifts with it. Ten thousand miles on the dial are really 9,795 on the road, so it over-counts by 205. At a true 70 mph the first turns 908 times a minute, the second 927.