155/65R18 vs 145/65R18
Going from one to the other changes the overall height by -2.0 %. That is inside the 3% band, so it fits and the speedometer stays close.
Side by side
| Measure | 155/65R18 | 145/65R18 | Change |
|---|---|---|---|
| Overall height | 25.9" | 25.4" | -2.0 % |
| Sidewall | 3.97" | 3.71" | -6.5 % |
| Section width | 155 mm | 145 mm | -6.5 % |
| Circumference | 81.5" | 79.9" | -2.0 % |
| Revs per mile | 778 | 793 | +2.0 % |
| Axle height | 12.97" | 12.71" | -6.5 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 155/65R18. Fit 145/65R18 and the needle stops telling the truth.
| Needle says | You are doing | Out by |
|---|---|---|
| 30 mph | 29.4 mph | -0.6 |
| 50 mph | 49.0 mph | -1.0 |
| 70 mph | 68.6 mph | -1.4 |
| 90 mph | 88.2 mph | -1.8 |
The odometer drifts with it. Ten thousand miles on the dial are really 9,803 on the road, so it over-counts by 197. At a true 70 mph the first turns 907 times a minute, the second 926.