125/80R14 vs 135/70R14
Going from one to the other changes the overall height by -2.0 %. That is inside the 3% band, so it fits and the speedometer stays close.
Side by side
| Measure | 125/80R14 | 135/70R14 | Change |
|---|---|---|---|
| Overall height | 21.9" | 21.4" | -2.0 % |
| Sidewall | 3.94" | 3.72" | -5.5 % |
| Section width | 125 mm | 135 mm | +8.0 % |
| Circumference | 68.7" | 67.4" | -2.0 % |
| Revs per mile | 922 | 941 | +2.0 % |
| Axle height | 10.94" | 10.72" | -5.5 mm |
| Approved rim width | 3.5–4.5" | 3.5–5" | 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 125/80R14. Fit 135/70R14 and the needle stops telling the truth.
| Needle says | You are doing | Out by |
|---|---|---|
| 30 mph | 29.4 mph | -0.6 |
| 50 mph | 49.0 mph | -1.0 |
| 70 mph | 68.6 mph | -1.4 |
| 90 mph | 88.2 mph | -1.8 |
The odometer drifts with it. Ten thousand miles on the dial are really 9,802 on the road, so it over-counts by 198. At a true 70 mph the first turns 1076 times a minute, the second 1097.