235/75R20 vs 215/85R20
Going from one to the other changes the overall height by +1.5 %. That is inside the 3% band, so it fits and the speedometer stays close.
Side by side
| Measure | 235/75R20 | 215/85R20 | Change |
|---|---|---|---|
| Overall height | 33.9" | 34.4" | +1.5 % |
| Sidewall | 6.94" | 7.19" | +3.7 % |
| Section width | 235 mm | 215 mm | -8.5 % |
| Circumference | 106.4" | 108.0" | +1.5 % |
| Revs per mile | 595 | 586 | -1.5 % |
| Axle height | 16.94" | 17.19" | +6.5 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 235/75R20. Fit 215/85R20 and the needle stops telling the truth.
| Needle says | You are doing | Out by |
|---|---|---|
| 30 mph | 30.5 mph | +0.5 |
| 50 mph | 50.8 mph | +0.8 |
| 70 mph | 71.1 mph | +1.1 |
| 90 mph | 91.4 mph | +1.4 |
The odometer drifts with it. Ten thousand miles on the dial are really 10,151 on the road, so it under-counts by 151. At a true 70 mph the first turns 695 times a minute, the second 684.