235/75R21 vs 215/85R21
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 | 235/75R21 | 215/85R21 | Change |
|---|---|---|---|
| Overall height | 34.9" | 35.4" | +1.5 % |
| Sidewall | 6.94" | 7.19" | +3.7 % |
| Section width | 235 mm | 215 mm | -8.5 % |
| Circumference | 109.6" | 111.2" | +1.5 % |
| Revs per mile | 578 | 570 | -1.5 % |
| Axle height | 17.44" | 17.69" | +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 235/75R21. Fit 215/85R21 and the needle stops telling the truth.
| Needle says | You are doing | Out by |
|---|---|---|
| 30 mph | 30.4 mph | +0.4 |
| 50 mph | 50.7 mph | +0.7 |
| 70 mph | 71.0 mph | +1.0 |
| 90 mph | 91.3 mph | +1.3 |
The odometer drifts with it. Ten thousand miles on the dial are really 10,147 on the road, so it under-counts by 147. At a true 70 mph the first turns 675 times a minute, the second 665.