215/85R20 vs 235/80R20
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 | 215/85R20 | 235/80R20 | Change |
|---|---|---|---|
| Overall height | 34.4" | 34.8" | +1.2 % |
| Sidewall | 7.19" | 7.40" | +2.9 % |
| Section width | 215 mm | 235 mm | +9.3 % |
| Circumference | 108.0" | 109.3" | +1.2 % |
| Revs per mile | 586 | 579 | -1.2 % |
| Axle height | 17.19" | 17.40" | +5.3 mm |
| Approved rim width | 6–8" | 6.5–8.5" | 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 215/85R20. Fit 235/80R20 and the needle stops telling the truth.
| Needle says | You are doing | Out by |
|---|---|---|
| 30 mph | 30.4 mph | +0.4 |
| 50 mph | 50.6 mph | +0.6 |
| 70 mph | 70.8 mph | +0.8 |
| 90 mph | 91.1 mph | +1.1 |
The odometer drifts with it. Ten thousand miles on the dial are really 10,120 on the road, so it under-counts by 120. At a true 70 mph the first turns 684 times a minute, the second 676.