165/65R15 vs 155/65R15
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 | 165/65R15 | 155/65R15 | Change |
|---|---|---|---|
| Overall height | 23.4" | 22.9" | -2.2 % |
| Sidewall | 4.22" | 3.97" | -6.1 % |
| Section width | 165 mm | 155 mm | -6.1 % |
| Circumference | 73.7" | 72.0" | -2.2 % |
| Revs per mile | 860 | 879 | +2.2 % |
| Axle height | 11.72" | 11.47" | -6.5 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 165/65R15. Fit 155/65R15 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 | 68.5 mph | -1.5 |
| 90 mph | 88.0 mph | -2.0 |
The odometer drifts with it. Ten thousand miles on the dial are really 9,782 on the road, so it over-counts by 218. At a true 70 mph the first turns 1004 times a minute, the second 1026.