215/80R18 vs 235/75R18
Going from one to the other changes the overall height by +1.1 %. That is inside the 3% band, so it fits and the speedometer stays close.
Side by side
| Measure | 215/80R18 | 235/75R18 | Change |
|---|---|---|---|
| Overall height | 31.5" | 31.9" | +1.1 % |
| Sidewall | 6.77" | 6.94" | +2.5 % |
| Section width | 215 mm | 235 mm | +9.3 % |
| Circumference | 99.1" | 100.1" | +1.1 % |
| Revs per mile | 639 | 633 | -1.1 % |
| Axle height | 15.77" | 15.94" | +4.3 mm |
| Approved rim width | 6–8" | 6.5–8.5" | overlap 6.5–8" |
Both are approved on a rim 6.5" to 8" wide. If yours is in that band, this is a tire change and nothing else.
What the speedometer will say
Calibrated for 215/80R18. Fit 235/75R18 and the needle stops telling the truth.
| Needle says | You are doing | Out by |
|---|---|---|
| 30 mph | 30.3 mph | +0.3 |
| 50 mph | 50.5 mph | +0.5 |
| 70 mph | 70.7 mph | +0.7 |
| 90 mph | 91.0 mph | +1.0 |
The odometer drifts with it. Ten thousand miles on the dial are really 10,106 on the road, so it under-counts by 106. At a true 70 mph the first turns 746 times a minute, the second 738.