155/65R18 vs 145/70R18
Going from one to the other changes the overall height by +0.2 %. That is inside the 3% band, so it fits and the speedometer stays close.
Side by side
| Measure | 155/65R18 | 145/70R18 | Change |
|---|---|---|---|
| Overall height | 25.9" | 26.0" | +0.2 % |
| Sidewall | 3.97" | 4.00" | +0.7 % |
| Section width | 155 mm | 145 mm | -6.5 % |
| Circumference | 81.5" | 81.7" | +0.2 % |
| Revs per mile | 778 | 776 | -0.2 % |
| Axle height | 12.97" | 13.00" | +0.8 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 145/70R18 and the needle stops telling the truth.
| Needle says | You are doing | Out by |
|---|---|---|
| 30 mph | 30.1 mph | +0.1 |
| 50 mph | 50.1 mph | +0.1 |
| 70 mph | 70.2 mph | +0.2 |
| 90 mph | 90.2 mph | +0.2 |
The odometer drifts with it. Ten thousand miles on the dial are really 10,023 on the road, so it under-counts by 23. At a true 70 mph the first turns 907 times a minute, the second 905.