125/75R18 vs 135/75R18
Going from one to the other changes the overall height by +2.3 %. That is inside the 3% band, so it fits and the speedometer stays close.
Side by side
| Measure | 125/75R18 | 135/75R18 | Change |
|---|---|---|---|
| Overall height | 25.4" | 26.0" | +2.3 % |
| Sidewall | 3.69" | 3.99" | +8.0 % |
| Section width | 125 mm | 135 mm | +8.0 % |
| Circumference | 79.7" | 81.6" | +2.3 % |
| Revs per mile | 795 | 777 | -2.3 % |
| Axle height | 12.69" | 12.99" | +7.5 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 125/75R18. Fit 135/75R18 and the needle stops telling the truth.
| Needle says | You are doing | Out by |
|---|---|---|
| 30 mph | 30.7 mph | +0.7 |
| 50 mph | 51.2 mph | +1.2 |
| 70 mph | 71.6 mph | +1.6 |
| 90 mph | 92.1 mph | +2.1 |
The odometer drifts with it. Ten thousand miles on the dial are really 10,233 on the road, so it under-counts by 233. At a true 70 mph the first turns 927 times a minute, the second 906.