155/55R15 vs 175/50R15
Going from one to the other changes the overall height by +0.8 %. That is inside the 3% band, so it fits and the speedometer stays close.
Side by side
| Measure | 155/55R15 | 175/50R15 | Change |
|---|---|---|---|
| Overall height | 21.7" | 21.9" | +0.8 % |
| Sidewall | 3.36" | 3.44" | +2.6 % |
| Section width | 155 mm | 175 mm | +12.9 % |
| Circumference | 68.2" | 68.8" | +0.8 % |
| Revs per mile | 929 | 921 | -0.8 % |
| Axle height | 10.86" | 10.94" | +2.3 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 155/55R15. Fit 175/50R15 and the needle stops telling the truth.
| Needle says | You are doing | Out by |
|---|---|---|
| 30 mph | 30.2 mph | +0.2 |
| 50 mph | 50.4 mph | +0.4 |
| 70 mph | 70.6 mph | +0.6 |
| 90 mph | 90.7 mph | +0.7 |
The odometer drifts with it. Ten thousand miles on the dial are really 10,082 on the road, so it under-counts by 82. At a true 70 mph the first turns 1084 times a minute, the second 1075.