175/80R15 vs 175/75R15
Going from one to the other changes the overall height by -2.6 %. That is inside the 3% band, so it fits and the speedometer stays close.
Side by side
| Measure | 175/80R15 | 175/75R15 | Change |
|---|---|---|---|
| Overall height | 26.0" | 25.3" | -2.6 % |
| Sidewall | 5.51" | 5.17" | -6.3 % |
| Section width | 175 mm | 175 mm | 0.0 % |
| Circumference | 81.8" | 79.6" | -2.6 % |
| Revs per mile | 775 | 796 | +2.6 % |
| Axle height | 13.01" | 12.67" | -8.8 mm |
| Approved rim width | 5–6" | 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 175/80R15. Fit 175/75R15 and the needle stops telling the truth.
| Needle says | You are doing | Out by |
|---|---|---|
| 30 mph | 29.2 mph | -0.8 |
| 50 mph | 48.7 mph | -1.3 |
| 70 mph | 68.1 mph | -1.9 |
| 90 mph | 87.6 mph | -2.4 |
The odometer drifts with it. Ten thousand miles on the dial are really 9,735 on the road, so it over-counts by 265. At a true 70 mph the first turns 904 times a minute, the second 929.