135/65R18 vs 125/75R18
Going from one to the other changes the overall height by +1.9 %. That is inside the 3% band, so it fits and the speedometer stays close.
Side by side
| Measure | 135/65R18 | 125/75R18 | Change |
|---|---|---|---|
| Overall height | 24.9" | 25.4" | +1.9 % |
| Sidewall | 3.45" | 3.69" | +6.8 % |
| Section width | 135 mm | 125 mm | -7.4 % |
| Circumference | 78.3" | 79.7" | +1.9 % |
| Revs per mile | 810 | 795 | -1.9 % |
| Axle height | 12.45" | 12.69" | +6.0 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 135/65R18. Fit 125/75R18 and the needle stops telling the truth.
| Needle says | You are doing | Out by |
|---|---|---|
| 30 mph | 30.6 mph | +0.6 |
| 50 mph | 50.9 mph | +0.9 |
| 70 mph | 71.3 mph | +1.3 |
| 90 mph | 91.7 mph | +1.7 |
The odometer drifts with it. Ten thousand miles on the dial are really 10,190 on the road, so it under-counts by 190. At a true 70 mph the first turns 945 times a minute, the second 927.