135/80R14 vs 125/80R14
Going from one to the other changes the overall height by -2.8 %. That is inside the 3% band, so it fits and the speedometer stays close.
Side by side
| Measure | 135/80R14 | 125/80R14 | Change |
|---|---|---|---|
| Overall height | 22.5" | 21.9" | -2.8 % |
| Sidewall | 4.25" | 3.94" | -7.4 % |
| Section width | 135 mm | 125 mm | -7.4 % |
| Circumference | 70.7" | 68.7" | -2.8 % |
| Revs per mile | 896 | 922 | +2.8 % |
| Axle height | 11.25" | 10.94" | -8.0 mm |
| Approved rim width | 3.5–5" | 3.5–4.5" | overlap 3.5–4.5" |
Both are approved on a rim 3.5" 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 135/80R14. Fit 125/80R14 and the needle stops telling the truth.
| Needle says | You are doing | Out by |
|---|---|---|
| 30 mph | 29.2 mph | -0.8 |
| 50 mph | 48.6 mph | -1.4 |
| 70 mph | 68.0 mph | -2.0 |
| 90 mph | 87.5 mph | -2.5 |
The odometer drifts with it. Ten thousand miles on the dial are really 9,720 on the road, so it over-counts by 280. At a true 70 mph the first turns 1046 times a minute, the second 1076.