155/70R18 vs 135/75R18
Going from one to the other changes the overall height by -2.2 %. That is inside the 3% band, so it fits and the speedometer stays close.
Side by side
| Measure | 155/70R18 | 135/75R18 | Change |
|---|---|---|---|
| Overall height | 26.5" | 26.0" | -2.2 % |
| Sidewall | 4.27" | 3.99" | -6.7 % |
| Section width | 155 mm | 135 mm | -12.9 % |
| Circumference | 83.4" | 81.6" | -2.2 % |
| Revs per mile | 760 | 777 | +2.2 % |
| Axle height | 13.27" | 12.99" | -7.3 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 155/70R18. Fit 135/75R18 and the needle stops telling the truth.
| Needle says | You are doing | Out by |
|---|---|---|
| 30 mph | 29.4 mph | -0.6 |
| 50 mph | 48.9 mph | -1.1 |
| 70 mph | 68.5 mph | -1.5 |
| 90 mph | 88.1 mph | -1.9 |
The odometer drifts with it. Ten thousand miles on the dial are really 9,785 on the road, so it over-counts by 215. At a true 70 mph the first turns 886 times a minute, the second 906.