145/75R18 vs 135/80R18
Going from one to the other changes the overall height by -0.2 %. That is inside the 3% band, so it fits and the speedometer stays close.
Side by side
| Measure | 145/75R18 | 135/80R18 | Change |
|---|---|---|---|
| Overall height | 26.6" | 26.5" | -0.2 % |
| Sidewall | 4.28" | 4.25" | -0.7 % |
| Section width | 145 mm | 135 mm | -6.9 % |
| Circumference | 83.5" | 83.3" | -0.2 % |
| Revs per mile | 759 | 761 | +0.2 % |
| Axle height | 13.28" | 13.25" | -0.8 mm |
| Approved rim width | 4–5.5" | 3.5–5" | overlap 4–5" |
Both are approved on a rim 4" to 5" wide. If yours is in that band, this is a tire change and nothing else.
What the speedometer will say
Calibrated for 145/75R18. Fit 135/80R18 and the needle stops telling the truth.
| Needle says | You are doing | Out by |
|---|---|---|
| 30 mph | 29.9 mph | -0.1 |
| 50 mph | 49.9 mph | -0.1 |
| 70 mph | 69.8 mph | -0.2 |
| 90 mph | 89.8 mph | -0.2 |
The odometer drifts with it. Ten thousand miles on the dial are really 9,978 on the road, so it over-counts by 22. At a true 70 mph the first turns 886 times a minute, the second 888.