135/75R18 vs 125/80R18
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 | 135/75R18 | 125/80R18 | Change |
|---|---|---|---|
| Overall height | 26.0" | 25.9" | -0.4 % |
| Sidewall | 3.99" | 3.94" | -1.2 % |
| Section width | 135 mm | 125 mm | -7.4 % |
| Circumference | 81.6" | 81.3" | -0.4 % |
| Revs per mile | 777 | 779 | +0.4 % |
| Axle height | 12.99" | 12.94" | -1.3 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 135/75R18. Fit 125/80R18 and the needle stops telling the truth.
| Needle says | You are doing | Out by |
|---|---|---|
| 30 mph | 29.9 mph | -0.1 |
| 50 mph | 49.8 mph | -0.2 |
| 70 mph | 69.7 mph | -0.3 |
| 90 mph | 89.7 mph | -0.3 |
The odometer drifts with it. Ten thousand miles on the dial are really 9,962 on the road, so it over-counts by 38. At a true 70 mph the first turns 906 times a minute, the second 909.