125/80R18 vs 145/75R18
Going from one to the other changes the overall height by +2.7 %. That is inside the 3% band, so it fits and the speedometer stays close.
Side by side
| Measure | 125/80R18 | 145/75R18 | Change |
|---|---|---|---|
| Overall height | 25.9" | 26.6" | +2.7 % |
| Sidewall | 3.94" | 4.28" | +8.8 % |
| Section width | 125 mm | 145 mm | +16.0 % |
| Circumference | 81.3" | 83.5" | +2.7 % |
| Revs per mile | 779 | 759 | -2.7 % |
| Axle height | 12.94" | 13.28" | +8.8 mm |
| Approved rim width | 3.5–4.5" | 4–5.5" | overlap 4–4.5" |
Both are approved on a rim 4" 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 145/75R18 and the needle stops telling the truth.
| Needle says | You are doing | Out by |
|---|---|---|
| 30 mph | 30.8 mph | +0.8 |
| 50 mph | 51.3 mph | +1.3 |
| 70 mph | 71.9 mph | +1.9 |
| 90 mph | 92.4 mph | +2.4 |
The odometer drifts with it. Ten thousand miles on the dial are really 10,266 on the road, so it under-counts by 266. At a true 70 mph the first turns 909 times a minute, the second 886.