125/80R18 vs 135/75R18
Going from one to the other changes the overall height by +0.4 %. That is inside the 3% band, so it fits and the speedometer stays close.
Side by side
| Measure | 125/80R18 | 135/75R18 | Change |
|---|---|---|---|
| Overall height | 25.9" | 26.0" | +0.4 % |
| Sidewall | 3.94" | 3.99" | +1.3 % |
| Section width | 125 mm | 135 mm | +8.0 % |
| Circumference | 81.3" | 81.6" | +0.4 % |
| Revs per mile | 779 | 777 | -0.4 % |
| Axle height | 12.94" | 12.99" | +1.3 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 125/80R18. Fit 135/75R18 and the needle stops telling the truth.
| Needle says | You are doing | Out by |
|---|---|---|
| 30 mph | 30.1 mph | +0.1 |
| 50 mph | 50.2 mph | +0.2 |
| 70 mph | 70.3 mph | +0.3 |
| 90 mph | 90.3 mph | +0.3 |
The odometer drifts with it. Ten thousand miles on the dial are really 10,038 on the road, so it under-counts by 38. At a true 70 mph the first turns 909 times a minute, the second 906.