205/70R13 vs 185/75R13
Going from one to the other changes the overall height by -1.5 %. That is inside the 3% band, so it fits and the speedometer stays close.
Side by side
| Measure | 205/70R13 | 185/75R13 | Change |
|---|---|---|---|
| Overall height | 24.3" | 23.9" | -1.5 % |
| Sidewall | 5.65" | 5.46" | -3.3 % |
| Section width | 205 mm | 185 mm | -9.8 % |
| Circumference | 76.3" | 75.2" | -1.5 % |
| Revs per mile | 830 | 843 | +1.5 % |
| Axle height | 12.15" | 11.96" | -4.8 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 205/70R13. Fit 185/75R13 and the needle stops telling the truth.
| Needle says | You are doing | Out by |
|---|---|---|
| 30 mph | 29.5 mph | -0.5 |
| 50 mph | 49.2 mph | -0.8 |
| 70 mph | 68.9 mph | -1.1 |
| 90 mph | 88.6 mph | -1.4 |
The odometer drifts with it. Ten thousand miles on the dial are really 9,846 on the road, so it over-counts by 154. At a true 70 mph the first turns 968 times a minute, the second 983.