155/75R18 vs 145/80R18
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 | 155/75R18 | 145/80R18 | Change |
|---|---|---|---|
| Overall height | 27.2" | 27.1" | -0.1 % |
| Sidewall | 4.58" | 4.57" | -0.2 % |
| Section width | 155 mm | 145 mm | -6.5 % |
| Circumference | 85.3" | 85.2" | -0.1 % |
| Revs per mile | 743 | 743 | +0.1 % |
| Axle height | 13.58" | 13.57" | -0.3 mm |
| Approved rim width | 4–5.5" | 4–5.5" | overlap 4–5.5" |
Both are approved on a rim 4" to 5.5" wide. If yours is in that band, this is a tire change and nothing else.
What the speedometer will say
Calibrated for 155/75R18. Fit 145/80R18 and the needle stops telling the truth.
| Needle says | You are doing | Out by |
|---|---|---|
| 30 mph | 30.0 mph | -0.0 |
| 50 mph | 50.0 mph | -0.0 |
| 70 mph | 69.9 mph | -0.1 |
| 90 mph | 89.9 mph | -0.1 |
The odometer drifts with it. Ten thousand miles on the dial are really 9,993 on the road, so it over-counts by 7. At a true 70 mph the first turns 867 times a minute, the second 867.