125/65R14 vs 145/55R14
Going from one to the other changes the overall height by -0.6 %. That is inside the 3% band, so it fits and the speedometer stays close.
Side by side
| Measure | 125/65R14 | 145/55R14 | Change |
|---|---|---|---|
| Overall height | 20.4" | 20.3" | -0.6 % |
| Sidewall | 3.20" | 3.14" | -1.8 % |
| Section width | 125 mm | 145 mm | +16.0 % |
| Circumference | 64.1" | 63.7" | -0.6 % |
| Revs per mile | 989 | 995 | +0.6 % |
| Axle height | 10.20" | 10.14" | -1.5 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 125/65R14. Fit 145/55R14 and the needle stops telling the truth.
| Needle says | You are doing | Out by |
|---|---|---|
| 30 mph | 29.8 mph | -0.2 |
| 50 mph | 49.7 mph | -0.3 |
| 70 mph | 69.6 mph | -0.4 |
| 90 mph | 89.5 mph | -0.5 |
The odometer drifts with it. Ten thousand miles on the dial are really 9,942 on the road, so it over-counts by 58. At a true 70 mph the first turns 1154 times a minute, the second 1160.