135/80R18 vs 155/70R18
Going from one to the other changes the overall height by +0.1 %. That is inside the 3% band, so it fits and the speedometer stays close.
Side by side
| Measure | 135/80R18 | 155/70R18 | Change |
|---|---|---|---|
| Overall height | 26.5" | 26.5" | +0.1 % |
| Sidewall | 4.25" | 4.27" | +0.5 % |
| Section width | 135 mm | 155 mm | +14.8 % |
| Circumference | 83.3" | 83.4" | +0.1 % |
| Revs per mile | 761 | 760 | -0.1 % |
| Axle height | 13.25" | 13.27" | +0.5 mm |
| Approved rim width | 3.5–5" | 4–5.5" | 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 135/80R18. Fit 155/70R18 and the needle stops telling the truth.
| Needle says | You are doing | Out by |
|---|---|---|
| 30 mph | 30.0 mph | +0.0 |
| 50 mph | 50.1 mph | +0.1 |
| 70 mph | 70.1 mph | +0.1 |
| 90 mph | 90.1 mph | +0.1 |
The odometer drifts with it. Ten thousand miles on the dial are really 10,015 on the road, so it under-counts by 15. At a true 70 mph the first turns 888 times a minute, the second 886.