145/80R18 vs 135/80R18
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 | 145/80R18 | 135/80R18 | Change |
|---|---|---|---|
| Overall height | 27.1" | 26.5" | -2.3 % |
| Sidewall | 4.57" | 4.25" | -6.9 % |
| Section width | 145 mm | 135 mm | -6.9 % |
| Circumference | 85.2" | 83.3" | -2.3 % |
| Revs per mile | 743 | 761 | +2.3 % |
| Axle height | 13.57" | 13.25" | -8.0 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 145/80R18. Fit 135/80R18 and the needle stops telling the truth.
| Needle says | You are doing | Out by |
|---|---|---|
| 30 mph | 29.3 mph | -0.7 |
| 50 mph | 48.8 mph | -1.2 |
| 70 mph | 68.4 mph | -1.6 |
| 90 mph | 87.9 mph | -2.1 |
The odometer drifts with it. Ten thousand miles on the dial are really 9,768 on the road, so it over-counts by 232. At a true 70 mph the first turns 867 times a minute, the second 888.