165/50R15 vs 155/50R15
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 | 165/50R15 | 155/50R15 | Change |
|---|---|---|---|
| Overall height | 21.5" | 21.1" | -1.8 % |
| Sidewall | 3.25" | 3.05" | -6.1 % |
| Section width | 165 mm | 155 mm | -6.1 % |
| Circumference | 67.5" | 66.3" | -1.8 % |
| Revs per mile | 938 | 956 | +1.8 % |
| Axle height | 10.75" | 10.55" | -5.0 mm |
| Approved rim width | 4.5–6" | 4–5.5" | overlap 4.5–5.5" |
Both are approved on a rim 4.5" to 5.5" wide. If yours is in that band, this is a tire change and nothing else.
What the speedometer will say
Calibrated for 165/50R15. Fit 155/50R15 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 9,817 on the road, so it over-counts by 183. At a true 70 mph the first turns 1095 times a minute, the second 1115.