245/80R20 vs 235/85R20
Going from one to the other changes the overall height by +0.8 %. That is inside the 3% band, so it fits and the speedometer stays close.
Side by side
| Measure | 245/80R20 | 235/85R20 | Change |
|---|---|---|---|
| Overall height | 35.4" | 35.7" | +0.8 % |
| Sidewall | 7.72" | 7.86" | +1.9 % |
| Section width | 245 mm | 235 mm | -4.1 % |
| Circumference | 111.3" | 112.2" | +0.8 % |
| Revs per mile | 569 | 564 | -0.8 % |
| Axle height | 17.72" | 17.86" | +3.8 mm |
| Approved rim width | 7–9" | 6.5–8.5" | overlap 7–8.5" |
Both are approved on a rim 7" 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 245/80R20. Fit 235/85R20 and the needle stops telling the truth.
| Needle says | You are doing | Out by |
|---|---|---|
| 30 mph | 30.3 mph | +0.3 |
| 50 mph | 50.4 mph | +0.4 |
| 70 mph | 70.6 mph | +0.6 |
| 90 mph | 90.8 mph | +0.8 |
The odometer drifts with it. Ten thousand miles on the dial are really 10,083 on the road, so it under-counts by 83. At a true 70 mph the first turns 664 times a minute, the second 659.