145/60R14 vs 125/75R14
Going from one to the other changes the overall height by +2.5 %. That is inside the 3% band, so it fits and the speedometer stays close.
Side by side
| Measure | 145/60R14 | 125/75R14 | Change |
|---|---|---|---|
| Overall height | 20.9" | 21.4" | +2.5 % |
| Sidewall | 3.43" | 3.69" | +7.8 % |
| Section width | 145 mm | 125 mm | -13.8 % |
| Circumference | 65.5" | 67.2" | +2.5 % |
| Revs per mile | 967 | 943 | -2.5 % |
| Axle height | 10.43" | 10.69" | +6.8 mm |
| Approved rim width | 4–5.5" | 3.5–4.5" | overlap 4–4.5" |
Both are approved on a rim 4" 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 145/60R14. Fit 125/75R14 and the needle stops telling the truth.
| Needle says | You are doing | Out by |
|---|---|---|
| 30 mph | 30.8 mph | +0.8 |
| 50 mph | 51.3 mph | +1.3 |
| 70 mph | 71.8 mph | +1.8 |
| 90 mph | 92.3 mph | +2.3 |
The odometer drifts with it. Ten thousand miles on the dial are really 10,255 on the road, so it under-counts by 255. At a true 70 mph the first turns 1128 times a minute, the second 1100.