225/80R20 vs 215/85R20
Going from one to the other changes the overall height by +0.6 %. That is inside the 3% band, so it fits and the speedometer stays close.
Side by side
| Measure | 225/80R20 | 215/85R20 | Change |
|---|---|---|---|
| Overall height | 34.2" | 34.4" | +0.6 % |
| Sidewall | 7.09" | 7.19" | +1.5 % |
| Section width | 225 mm | 215 mm | -4.4 % |
| Circumference | 107.4" | 108.0" | +0.6 % |
| Revs per mile | 590 | 586 | -0.6 % |
| Axle height | 17.09" | 17.19" | +2.8 mm |
| Approved rim width | 6–8.5" | 6–8" | overlap 6–8" |
Both are approved on a rim 6" to 8" wide. If yours is in that band, this is a tire change and nothing else.
What the speedometer will say
Calibrated for 225/80R20. Fit 215/85R20 and the needle stops telling the truth.
| Needle says | You are doing | Out by |
|---|---|---|
| 30 mph | 30.2 mph | +0.2 |
| 50 mph | 50.3 mph | +0.3 |
| 70 mph | 70.4 mph | +0.4 |
| 90 mph | 90.6 mph | +0.6 |
The odometer drifts with it. Ten thousand miles on the dial are really 10,063 on the road, so it under-counts by 63. At a true 70 mph the first turns 689 times a minute, the second 684.