145/80R18 vs 145/75R18
Going from one to the other changes the overall height by -2.1 %. That is inside the 3% band, so it fits and the speedometer stays close.
Side by side
| Measure | 145/80R18 | 145/75R18 | Change |
|---|---|---|---|
| Overall height | 27.1" | 26.6" | -2.1 % |
| Sidewall | 4.57" | 4.28" | -6.3 % |
| Section width | 145 mm | 145 mm | 0.0 % |
| Circumference | 85.2" | 83.5" | -2.1 % |
| Revs per mile | 743 | 759 | +2.1 % |
| Axle height | 13.57" | 13.28" | -7.3 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 145/80R18. Fit 145/75R18 and the needle stops telling the truth.
| Needle says | You are doing | Out by |
|---|---|---|
| 30 mph | 29.4 mph | -0.6 |
| 50 mph | 48.9 mph | -1.1 |
| 70 mph | 68.5 mph | -1.5 |
| 90 mph | 88.1 mph | -1.9 |
The odometer drifts with it. Ten thousand miles on the dial are really 9,790 on the road, so it over-counts by 210. At a true 70 mph the first turns 867 times a minute, the second 886.