255/75R23 vs 235/85R23
Going from one to the other changes the overall height by +1.8 %. That is inside the 3% band, so it fits and the speedometer stays close.
Side by side
| Measure | 255/75R23 | 235/85R23 | Change |
|---|---|---|---|
| Overall height | 38.1" | 38.7" | +1.8 % |
| Sidewall | 7.53" | 7.86" | +4.4 % |
| Section width | 255 mm | 235 mm | -7.8 % |
| Circumference | 119.6" | 121.7" | +1.8 % |
| Revs per mile | 530 | 521 | -1.8 % |
| Axle height | 19.03" | 19.36" | +8.5 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 255/75R23. Fit 235/85R23 and the needle stops telling the truth.
| Needle says | You are doing | Out by |
|---|---|---|
| 30 mph | 30.5 mph | +0.5 |
| 50 mph | 50.9 mph | +0.9 |
| 70 mph | 71.2 mph | +1.2 |
| 90 mph | 91.6 mph | +1.6 |
The odometer drifts with it. Ten thousand miles on the dial are really 10,176 on the road, so it under-counts by 176. At a true 70 mph the first turns 618 times a minute, the second 608.