235/75R23 vs 255/65R23
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 | 235/75R23 | 255/65R23 | Change |
|---|---|---|---|
| Overall height | 36.9" | 36.1" | -2.2 % |
| Sidewall | 6.94" | 6.53" | -6.0 % |
| Section width | 235 mm | 255 mm | +8.5 % |
| Circumference | 115.9" | 113.3" | -2.2 % |
| Revs per mile | 547 | 559 | +2.2 % |
| Axle height | 18.44" | 18.03" | -10.5 mm |
| Approved rim width | 6.5–8.5" | 7–9.5" | 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 235/75R23. Fit 255/65R23 and the needle stops telling the truth.
| Needle says | You are doing | Out by |
|---|---|---|
| 30 mph | 29.3 mph | -0.7 |
| 50 mph | 48.9 mph | -1.1 |
| 70 mph | 68.4 mph | -1.6 |
| 90 mph | 88.0 mph | -2.0 |
The odometer drifts with it. Ten thousand miles on the dial are really 9,776 on the road, so it over-counts by 224. At a true 70 mph the first turns 638 times a minute, the second 653.