175/50R15 vs 165/50R15
Going from one to the other changes the overall height by -1.8 %. That is inside the 3% band, so it fits and the speedometer stays close.
Side by side
| Measure | 175/50R15 | 165/50R15 | Change |
|---|---|---|---|
| Overall height | 21.9" | 21.5" | -1.8 % |
| Sidewall | 3.44" | 3.25" | -5.7 % |
| Section width | 175 mm | 165 mm | -5.7 % |
| Circumference | 68.8" | 67.5" | -1.8 % |
| Revs per mile | 921 | 938 | +1.8 % |
| Axle height | 10.94" | 10.75" | -5.0 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 175/50R15. Fit 165/50R15 and the needle stops telling the truth.
| Needle says | You are doing | Out by |
|---|---|---|
| 30 mph | 29.5 mph | -0.5 |
| 50 mph | 49.1 mph | -0.9 |
| 70 mph | 68.7 mph | -1.3 |
| 90 mph | 88.4 mph | -1.6 |
The odometer drifts with it. Ten thousand miles on the dial are really 9,820 on the road, so it over-counts by 180. At a true 70 mph the first turns 1075 times a minute, the second 1095.