175/70R15 vs 165/70R15
Going from one to the other changes the overall height by -2.2 %. That is inside the 3% band, so it fits and the speedometer stays close.
Side by side
| Measure | 175/70R15 | 165/70R15 | Change |
|---|---|---|---|
| Overall height | 24.6" | 24.1" | -2.2 % |
| Sidewall | 4.82" | 4.55" | -5.7 % |
| Section width | 175 mm | 165 mm | -5.7 % |
| Circumference | 77.4" | 75.7" | -2.2 % |
| Revs per mile | 818 | 837 | +2.2 % |
| Axle height | 12.32" | 12.05" | -7.0 mm |
| Approved rim width | 5–6" | 4.5–6" | 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 175/70R15. Fit 165/70R15 and the needle stops telling the truth.
| Needle says | You are doing | Out by |
|---|---|---|
| 30 mph | 29.3 mph | -0.7 |
| 50 mph | 48.9 mph | -1.1 |
| 70 mph | 68.4 mph | -1.6 |
| 90 mph | 88.0 mph | -2.0 |
The odometer drifts with it. Ten thousand miles on the dial are really 9,776 on the road, so it over-counts by 224. At a true 70 mph the first turns 955 times a minute, the second 977.