185/65R15 vs 175/65R15
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 | 185/65R15 | 175/65R15 | Change |
|---|---|---|---|
| Overall height | 24.5" | 24.0" | -2.1 % |
| Sidewall | 4.73" | 4.48" | -5.4 % |
| Section width | 185 mm | 175 mm | -5.4 % |
| Circumference | 76.9" | 75.3" | -2.1 % |
| Revs per mile | 824 | 842 | +2.1 % |
| Axle height | 12.23" | 11.98" | -6.5 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 185/65R15. Fit 175/65R15 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 | 68.5 mph | -1.5 |
| 90 mph | 88.1 mph | -1.9 |
The odometer drifts with it. Ten thousand miles on the dial are really 9,791 on the road, so it over-counts by 209. At a true 70 mph the first turns 962 times a minute, the second 982.