145/65R14 vs 125/75R14
Going from one to the other changes the overall height by -0.2 %. That is inside the 3% band, so it fits and the speedometer stays close.
Side by side
| Measure | 145/65R14 | 125/75R14 | Change |
|---|---|---|---|
| Overall height | 21.4" | 21.4" | -0.2 % |
| Sidewall | 3.71" | 3.69" | -0.5 % |
| Section width | 145 mm | 125 mm | -13.8 % |
| Circumference | 67.3" | 67.2" | -0.2 % |
| Revs per mile | 941 | 943 | +0.2 % |
| Axle height | 10.71" | 10.69" | -0.5 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 145/65R14. Fit 125/75R14 and the needle stops telling the truth.
| Needle says | You are doing | Out by |
|---|---|---|
| 30 mph | 29.9 mph | -0.1 |
| 50 mph | 49.9 mph | -0.1 |
| 70 mph | 69.9 mph | -0.1 |
| 90 mph | 89.8 mph | -0.2 |
The odometer drifts with it. Ten thousand miles on the dial are really 9,982 on the road, so it over-counts by 18. At a true 70 mph the first turns 1098 times a minute, the second 1100.