235/80R18 vs 235/75R18
Going from one to the other changes the overall height by -2.8 %. That is inside the 3% band, so it fits and the speedometer stays close.
Side by side
| Measure | 235/80R18 | 235/75R18 | Change |
|---|---|---|---|
| Overall height | 32.8" | 31.9" | -2.8 % |
| Sidewall | 7.40" | 6.94" | -6.3 % |
| Section width | 235 mm | 235 mm | 0.0 % |
| Circumference | 103.1" | 100.1" | -2.8 % |
| Revs per mile | 615 | 633 | +2.8 % |
| Axle height | 16.40" | 15.94" | -11.8 mm |
| Approved rim width | 6.5–8.5" | 6.5–8.5" | overlap 6.5–8.5" |
Both are approved on a rim 6.5" to 8.5" wide. If yours is in that band, this is a tire change and nothing else.
What the speedometer will say
Calibrated for 235/80R18. Fit 235/75R18 and the needle stops telling the truth.
| Needle says | You are doing | Out by |
|---|---|---|
| 30 mph | 29.2 mph | -0.8 |
| 50 mph | 48.6 mph | -1.4 |
| 70 mph | 68.0 mph | -2.0 |
| 90 mph | 87.5 mph | -2.5 |
The odometer drifts with it. Ten thousand miles on the dial are really 9,718 on the road, so it over-counts by 282. At a true 70 mph the first turns 717 times a minute, the second 738.