185/75R15 vs 175/80R15
Going from one to the other changes the overall height by +0.4 %. That is inside the 3% band, so it fits and the speedometer stays close.
Side by side
| Measure | 185/75R15 | 175/80R15 | Change |
|---|---|---|---|
| Overall height | 25.9" | 26.0" | +0.4 % |
| Sidewall | 5.46" | 5.51" | +0.9 % |
| Section width | 185 mm | 175 mm | -5.4 % |
| Circumference | 81.4" | 81.8" | +0.4 % |
| Revs per mile | 778 | 775 | -0.4 % |
| Axle height | 12.96" | 13.01" | +1.3 mm |
| Approved rim width | 5–6.5" | 5–6" | overlap 5–6" |
Both are approved on a rim 5" to 6" wide. If yours is in that band, this is a tire change and nothing else.
What the speedometer will say
Calibrated for 185/75R15. Fit 175/80R15 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 | 90.3 mph | +0.3 |
The odometer drifts with it. Ten thousand miles on the dial are really 10,038 on the road, so it under-counts by 38. At a true 70 mph the first turns 908 times a minute, the second 904.