165/85R15 vs 155/85R15
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 | 165/85R15 | 155/85R15 | Change |
|---|---|---|---|
| Overall height | 26.0" | 25.4" | -2.6 % |
| Sidewall | 5.52" | 5.19" | -6.1 % |
| Section width | 165 mm | 155 mm | -6.1 % |
| Circumference | 81.8" | 79.7" | -2.6 % |
| Revs per mile | 774 | 795 | +2.6 % |
| Axle height | 13.02" | 12.69" | -8.5 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 165/85R15. Fit 155/85R15 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 | 87.7 mph | -2.3 |
The odometer drifts with it. Ten thousand miles on the dial are really 9,743 on the road, so it over-counts by 257. At a true 70 mph the first turns 903 times a minute, the second 927.