125/75R15 vs 135/70R15
Going from one to the other changes the overall height by +0.3 %. That is inside the 3% band, so it fits and the speedometer stays close.
Side by side
| Measure | 125/75R15 | 135/70R15 | Change |
|---|---|---|---|
| Overall height | 22.4" | 22.4" | +0.3 % |
| Sidewall | 3.69" | 3.72" | +0.8 % |
| Section width | 125 mm | 135 mm | +8.0 % |
| Circumference | 70.3" | 70.5" | +0.3 % |
| Revs per mile | 901 | 899 | -0.3 % |
| Axle height | 11.19" | 11.22" | +0.8 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 125/75R15. Fit 135/70R15 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 | 70.2 mph | +0.2 |
| 90 mph | 90.2 mph | +0.2 |
The odometer drifts with it. Ten thousand miles on the dial are really 10,026 on the road, so it under-counts by 26. At a true 70 mph the first turns 1051 times a minute, the second 1049.