125/75R14 vs 145/65R14
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 | 125/75R14 | 145/65R14 | Change |
|---|---|---|---|
| Overall height | 21.4" | 21.4" | +0.2 % |
| Sidewall | 3.69" | 3.71" | +0.5 % |
| Section width | 125 mm | 145 mm | +16.0 % |
| Circumference | 67.2" | 67.3" | +0.2 % |
| Revs per mile | 943 | 941 | -0.2 % |
| Axle height | 10.69" | 10.71" | +0.5 mm |
| Approved rim width | 3.5–4.5" | 4–5.5" | 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 125/75R14. Fit 145/65R14 and the needle stops telling the truth.
| Needle says | You are doing | Out by |
|---|---|---|
| 30 mph | 30.1 mph | +0.1 |
| 50 mph | 50.1 mph | +0.1 |
| 70 mph | 70.1 mph | +0.1 |
| 90 mph | 90.2 mph | +0.2 |
The odometer drifts with it. Ten thousand miles on the dial are really 10,018 on the road, so it under-counts by 18. At a true 70 mph the first turns 1100 times a minute, the second 1098.