225/80R20 vs 205/85R20
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 | 225/80R20 | 205/85R20 | Change |
|---|---|---|---|
| Overall height | 34.2" | 33.7" | -1.3 % |
| Sidewall | 7.09" | 6.86" | -3.2 % |
| Section width | 225 mm | 205 mm | -8.9 % |
| Circumference | 107.4" | 105.9" | -1.3 % |
| Revs per mile | 590 | 598 | +1.3 % |
| Axle height | 17.09" | 16.86" | -5.8 mm |
| Approved rim width | 6–8.5" | 5.5–7.5" | overlap 6–7.5" |
Both are approved on a rim 6" to 7.5" 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 205/85R20 and the needle stops telling the truth.
| Needle says | You are doing | Out by |
|---|---|---|
| 30 mph | 29.6 mph | -0.4 |
| 50 mph | 49.3 mph | -0.7 |
| 70 mph | 69.1 mph | -0.9 |
| 90 mph | 88.8 mph | -1.2 |
The odometer drifts with it. Ten thousand miles on the dial are really 9,868 on the road, so it over-counts by 132. At a true 70 mph the first turns 689 times a minute, the second 698.