235/80R18 vs 215/85R18
Going from one to the other changes the overall height by -1.3 %. That is inside the 3% band, so it fits and the speedometer stays close.
Side by side
| Measure | 235/80R18 | 215/85R18 | Change |
|---|---|---|---|
| Overall height | 32.8" | 32.4" | -1.3 % |
| Sidewall | 7.40" | 7.19" | -2.8 % |
| Section width | 235 mm | 215 mm | -8.5 % |
| Circumference | 103.1" | 101.8" | -1.3 % |
| Revs per mile | 615 | 623 | +1.3 % |
| Axle height | 16.40" | 16.19" | -5.3 mm |
| Approved rim width | 6.5–8.5" | 6–8" | 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 235/80R18. Fit 215/85R18 and the needle stops telling the truth.
| Needle says | You are doing | Out by |
|---|---|---|
| 30 mph | 29.6 mph | -0.4 |
| 50 mph | 49.4 mph | -0.6 |
| 70 mph | 69.1 mph | -0.9 |
| 90 mph | 88.9 mph | -1.1 |
The odometer drifts with it. Ten thousand miles on the dial are really 9,874 on the road, so it over-counts by 126. At a true 70 mph the first turns 717 times a minute, the second 726.