255/70R20 vs 235/80R20
Going from one to the other changes the overall height by +2.2 %. That is inside the 3% band, so it fits and the speedometer stays close.
Side by side
| Measure | 255/70R20 | 235/80R20 | Change |
|---|---|---|---|
| Overall height | 34.1" | 34.8" | +2.2 % |
| Sidewall | 7.03" | 7.40" | +5.3 % |
| Section width | 255 mm | 235 mm | -7.8 % |
| Circumference | 107.0" | 109.3" | +2.2 % |
| Revs per mile | 592 | 579 | -2.2 % |
| Axle height | 17.03" | 17.40" | +9.5 mm |
| Approved rim width | 7–9.5" | 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 255/70R20. Fit 235/80R20 and the needle stops telling the truth.
| Needle says | You are doing | Out by |
|---|---|---|
| 30 mph | 30.7 mph | +0.7 |
| 50 mph | 51.1 mph | +1.1 |
| 70 mph | 71.5 mph | +1.5 |
| 90 mph | 92.0 mph | +2.0 |
The odometer drifts with it. Ten thousand miles on the dial are really 10,220 on the road, so it under-counts by 220. At a true 70 mph the first turns 691 times a minute, the second 676.