155/85R15 vs 175/75R15
Going from one to the other changes the overall height by -0.2 %. That is inside the 3% band, so it fits and the speedometer stays close.
Side by side
| Measure | 155/85R15 | 175/75R15 | Change |
|---|---|---|---|
| Overall height | 25.4" | 25.3" | -0.2 % |
| Sidewall | 5.19" | 5.17" | -0.4 % |
| Section width | 155 mm | 175 mm | +12.9 % |
| Circumference | 79.7" | 79.6" | -0.2 % |
| Revs per mile | 795 | 796 | +0.2 % |
| Axle height | 12.69" | 12.67" | -0.5 mm |
| Approved rim width | 4–5.5" | 5–6" | 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 155/85R15. Fit 175/75R15 and the needle stops telling the truth.
| Needle says | You are doing | Out by |
|---|---|---|
| 30 mph | 30.0 mph | -0.0 |
| 50 mph | 49.9 mph | -0.1 |
| 70 mph | 69.9 mph | -0.1 |
| 90 mph | 89.9 mph | -0.1 |
The odometer drifts with it. Ten thousand miles on the dial are really 9,984 on the road, so it over-counts by 16. At a true 70 mph the first turns 927 times a minute, the second 929.