145/70R16 vs 125/80R16
Going from one to the other changes the overall height by -0.5 %. That is inside the 3% band, so it fits and the speedometer stays close.
Side by side
| Measure | 145/70R16 | 125/80R16 | Change |
|---|---|---|---|
| Overall height | 24.0" | 23.9" | -0.5 % |
| Sidewall | 4.00" | 3.94" | -1.5 % |
| Section width | 145 mm | 125 mm | -13.8 % |
| Circumference | 75.4" | 75.0" | -0.5 % |
| Revs per mile | 841 | 845 | +0.5 % |
| Axle height | 12.00" | 11.94" | -1.5 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 145/70R16. Fit 125/80R16 and the needle stops telling the truth.
| Needle says | You are doing | Out by |
|---|---|---|
| 30 mph | 29.9 mph | -0.1 |
| 50 mph | 49.8 mph | -0.2 |
| 70 mph | 69.7 mph | -0.3 |
| 90 mph | 89.6 mph | -0.4 |
The odometer drifts with it. Ten thousand miles on the dial are really 9,951 on the road, so it over-counts by 49. At a true 70 mph the first turns 981 times a minute, the second 986.