215/85R18 vs 235/75R18
Going from one to the other changes the overall height by -1.6 %. That is inside the 3% band, so it fits and the speedometer stays close.
Side by side
| Measure | 215/85R18 | 235/75R18 | Change |
|---|---|---|---|
| Overall height | 32.4" | 31.9" | -1.6 % |
| Sidewall | 7.19" | 6.94" | -3.6 % |
| Section width | 215 mm | 235 mm | +9.3 % |
| Circumference | 101.8" | 100.1" | -1.6 % |
| Revs per mile | 623 | 633 | +1.6 % |
| Axle height | 16.19" | 15.94" | -6.5 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 215/85R18. Fit 235/75R18 and the needle stops telling the truth.
| Needle says | You are doing | Out by |
|---|---|---|
| 30 mph | 29.5 mph | -0.5 |
| 50 mph | 49.2 mph | -0.8 |
| 70 mph | 68.9 mph | -1.1 |
| 90 mph | 88.6 mph | -1.4 |
The odometer drifts with it. Ten thousand miles on the dial are really 9,842 on the road, so it over-counts by 158. At a true 70 mph the first turns 726 times a minute, the second 738.