175/70R15 vs 155/80R15
Going from one to the other changes the overall height by +0.5 %. That is inside the 3% band, so it fits and the speedometer stays close.
Side by side
| Measure | 175/70R15 | 155/80R15 | Change |
|---|---|---|---|
| Overall height | 24.6" | 24.8" | +0.5 % |
| Sidewall | 4.82" | 4.88" | +1.2 % |
| Section width | 175 mm | 155 mm | -11.4 % |
| Circumference | 77.4" | 77.8" | +0.5 % |
| Revs per mile | 818 | 814 | -0.5 % |
| Axle height | 12.32" | 12.38" | +1.5 mm |
| Approved rim width | 5–6" | 4–5.5" | overlap 5–5.5" |
Both are approved on a rim 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 175/70R15. Fit 155/80R15 and the needle stops telling the truth.
| Needle says | You are doing | Out by |
|---|---|---|
| 30 mph | 30.1 mph | +0.1 |
| 50 mph | 50.2 mph | +0.2 |
| 70 mph | 70.3 mph | +0.3 |
| 90 mph | 90.4 mph | +0.4 |
The odometer drifts with it. Ten thousand miles on the dial are really 10,048 on the road, so it under-counts by 48. At a true 70 mph the first turns 955 times a minute, the second 950.