255/75R23 vs 235/80R23
Going from one to the other changes the overall height by -0.7 %. That is inside the 3% band, so it fits and the speedometer stays close.
Side by side
| Measure | 255/75R23 | 235/80R23 | Change |
|---|---|---|---|
| Overall height | 38.1" | 37.8" | -0.7 % |
| Sidewall | 7.53" | 7.40" | -1.7 % |
| Section width | 255 mm | 235 mm | -7.8 % |
| Circumference | 119.6" | 118.8" | -0.7 % |
| Revs per mile | 530 | 534 | +0.7 % |
| Axle height | 19.03" | 18.90" | -3.3 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 235/80R23 and the needle stops telling the truth.
| Needle says | You are doing | Out by |
|---|---|---|
| 30 mph | 29.8 mph | -0.2 |
| 50 mph | 49.7 mph | -0.3 |
| 70 mph | 69.5 mph | -0.5 |
| 90 mph | 89.4 mph | -0.6 |
The odometer drifts with it. Ten thousand miles on the dial are really 9,933 on the road, so it over-counts by 67. At a true 70 mph the first turns 618 times a minute, the second 622.