235/85R20 vs 235/80R20
Going from one to the other changes the overall height by -2.6 %. That is inside the 3% band, so it fits and the speedometer stays close.
Side by side
| Measure | 235/85R20 | 235/80R20 | Change |
|---|---|---|---|
| Overall height | 35.7" | 34.8" | -2.6 % |
| Sidewall | 7.86" | 7.40" | -5.9 % |
| Section width | 235 mm | 235 mm | 0.0 % |
| Circumference | 112.2" | 109.3" | -2.6 % |
| Revs per mile | 564 | 579 | +2.6 % |
| Axle height | 17.86" | 17.40" | -11.8 mm |
| Approved rim width | 6.5–8.5" | 6.5–8.5" | overlap 6.5–8.5" |
Both are approved on a rim 6.5" to 8.5" wide. If yours is in that band, this is a tire change and nothing else.
What the speedometer will say
Calibrated for 235/85R20. Fit 235/80R20 and the needle stops telling the truth.
| Needle says | You are doing | Out by |
|---|---|---|
| 30 mph | 29.2 mph | -0.8 |
| 50 mph | 48.7 mph | -1.3 |
| 70 mph | 68.2 mph | -1.8 |
| 90 mph | 87.7 mph | -2.3 |
The odometer drifts with it. Ten thousand miles on the dial are really 9,741 on the road, so it over-counts by 259. At a true 70 mph the first turns 659 times a minute, the second 676.