125/80R17 vs 125/75R17
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 | 125/80R17 | 125/75R17 | Change |
|---|---|---|---|
| Overall height | 24.9" | 24.4" | -2.0 % |
| Sidewall | 3.94" | 3.69" | -6.3 % |
| Section width | 125 mm | 125 mm | 0.0 % |
| Circumference | 78.1" | 76.6" | -2.0 % |
| Revs per mile | 811 | 827 | +2.0 % |
| Axle height | 12.44" | 12.19" | -6.3 mm |
| Approved rim width | 3.5–4.5" | 3.5–4.5" | overlap 3.5–4.5" |
Both are approved on a rim 3.5" to 4.5" wide. If yours is in that band, this is a tire change and nothing else.
What the speedometer will say
Calibrated for 125/80R17. Fit 125/75R17 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 9,802 on the road, so it over-counts by 198. At a true 70 mph the first turns 946 times a minute, the second 965.