235/80R20 vs 215/85R20
Going from one to the other changes the overall height by -1.2 %. That is inside the 3% band, so it fits and the speedometer stays close.
Side by side
| Measure | 235/80R20 | 215/85R20 | Change |
|---|---|---|---|
| Overall height | 34.8" | 34.4" | -1.2 % |
| Sidewall | 7.40" | 7.19" | -2.8 % |
| Section width | 235 mm | 215 mm | -8.5 % |
| Circumference | 109.3" | 108.0" | -1.2 % |
| Revs per mile | 579 | 586 | +1.2 % |
| Axle height | 17.40" | 17.19" | -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 235/80R20. Fit 215/85R20 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 | 69.2 mph | -0.8 |
| 90 mph | 88.9 mph | -1.1 |
The odometer drifts with it. Ten thousand miles on the dial are really 9,881 on the road, so it over-counts by 119. At a true 70 mph the first turns 676 times a minute, the second 684.