175/70R15 vs 155/75R15
Going from one to the other changes the overall height by -2.0 %. That is inside the 3% band, so it fits and the speedometer stays close.
Side by side
| Measure | 175/70R15 | 155/75R15 | Change |
|---|---|---|---|
| Overall height | 24.6" | 24.2" | -2.0 % |
| Sidewall | 4.82" | 4.58" | -5.1 % |
| Section width | 175 mm | 155 mm | -11.4 % |
| Circumference | 77.4" | 75.9" | -2.0 % |
| Revs per mile | 818 | 835 | +2.0 % |
| Axle height | 12.32" | 12.08" | -6.3 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 155/75R15 and the needle stops telling the truth.
| Needle says | You are doing | Out by |
|---|---|---|
| 30 mph | 29.4 mph | -0.6 |
| 50 mph | 49.0 mph | -1.0 |
| 70 mph | 68.6 mph | -1.4 |
| 90 mph | 88.2 mph | -1.8 |
The odometer drifts with it. Ten thousand miles on the dial are really 9,800 on the road, so it over-counts by 200. At a true 70 mph the first turns 955 times a minute, the second 974.
What drivers reported
175/70R15 carries 2 complaints. Nobody has filed one naming 155/75R15.