155/65R18 vs 145/75R18
Going from one to the other changes the overall height by +2.4 %. That is inside the 3% band, so it fits and the speedometer stays close.
Side by side
| Measure | 155/65R18 | 145/75R18 | Change |
|---|---|---|---|
| Overall height | 25.9" | 26.6" | +2.4 % |
| Sidewall | 3.97" | 4.28" | +7.9 % |
| Section width | 155 mm | 145 mm | -6.5 % |
| Circumference | 81.5" | 83.5" | +2.4 % |
| Revs per mile | 778 | 759 | -2.4 % |
| Axle height | 12.97" | 13.28" | +8.0 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 145/75R18 and the needle stops telling the truth.
| Needle says | You are doing | Out by |
|---|---|---|
| 30 mph | 30.7 mph | +0.7 |
| 50 mph | 51.2 mph | +1.2 |
| 70 mph | 71.7 mph | +1.7 |
| 90 mph | 92.2 mph | +2.2 |
The odometer drifts with it. Ten thousand miles on the dial are really 10,243 on the road, so it under-counts by 243. At a true 70 mph the first turns 907 times a minute, the second 886.