205/80R20 vs 205/85R20
Going from one to the other changes the overall height by +2.5 %. That is inside the 3% band, so it fits and the speedometer stays close.
Side by side
| Measure | 205/80R20 | 205/85R20 | Change |
|---|---|---|---|
| Overall height | 32.9" | 33.7" | +2.5 % |
| Sidewall | 6.46" | 6.86" | +6.3 % |
| Section width | 205 mm | 205 mm | 0.0 % |
| Circumference | 103.4" | 105.9" | +2.5 % |
| Revs per mile | 613 | 598 | -2.5 % |
| Axle height | 16.46" | 16.86" | +10.3 mm |
| Approved rim width | 5.5–7.5" | 5.5–7.5" | overlap 5.5–7.5" |
Both are approved on a rim 5.5" 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 205/80R20. Fit 205/85R20 and the needle stops telling the truth.
| Needle says | You are doing | Out by |
|---|---|---|
| 30 mph | 30.7 mph | +0.7 |
| 50 mph | 51.2 mph | +1.2 |
| 70 mph | 71.7 mph | +1.7 |
| 90 mph | 92.2 mph | +2.2 |
The odometer drifts with it. Ten thousand miles on the dial are really 10,245 on the road, so it under-counts by 245. At a true 70 mph the first turns 715 times a minute, the second 698.