125/80R14 vs 125/75R14
Going from one to the other changes the overall height by -2.2 %. That is inside the 3% band, so it fits and the speedometer stays close.
Side by side
| Measure | 125/80R14 | 125/75R14 | Change |
|---|---|---|---|
| Overall height | 21.9" | 21.4" | -2.2 % |
| Sidewall | 3.94" | 3.69" | -6.3 % |
| Section width | 125 mm | 125 mm | 0.0 % |
| Circumference | 68.7" | 67.2" | -2.2 % |
| Revs per mile | 922 | 943 | +2.2 % |
| Axle height | 10.94" | 10.69" | -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 125/80R14. Fit 125/75R14 and the needle stops telling the truth.
| Needle says | You are doing | Out by |
|---|---|---|
| 30 mph | 29.3 mph | -0.7 |
| 50 mph | 48.9 mph | -1.1 |
| 70 mph | 68.4 mph | -1.6 |
| 90 mph | 88.0 mph | -2.0 |
The odometer drifts with it. Ten thousand miles on the dial are really 9,775 on the road, so it over-counts by 225. At a true 70 mph the first turns 1076 times a minute, the second 1100.