185/85R13 vs 175/85R13
Going from one to the other changes the overall height by -2.6 %. That is inside the 3% band, so it fits and the speedometer stays close.
Side by side
| Measure | 185/85R13 | 175/85R13 | Change |
|---|---|---|---|
| Overall height | 25.4" | 24.7" | -2.6 % |
| Sidewall | 6.19" | 5.86" | -5.4 % |
| Section width | 185 mm | 175 mm | -5.4 % |
| Circumference | 79.7" | 77.6" | -2.6 % |
| Revs per mile | 795 | 816 | +2.6 % |
| Axle height | 12.69" | 12.36" | -8.5 mm |
| Approved rim width | 5–6.5" | 5–6" | overlap 5–6" |
Both are approved on a rim 5" to 6" wide. If yours is in that band, this is a tire change and nothing else.
What the speedometer will say
Calibrated for 185/85R13. Fit 175/85R13 and the needle stops telling the truth.
| Needle says | You are doing | Out by |
|---|---|---|
| 30 mph | 29.2 mph | -0.8 |
| 50 mph | 48.7 mph | -1.3 |
| 70 mph | 68.2 mph | -1.8 |
| 90 mph | 87.6 mph | -2.4 |
The odometer drifts with it. Ten thousand miles on the dial are really 9,736 on the road, so it over-counts by 264. At a true 70 mph the first turns 927 times a minute, the second 952.