235/45R23 vs 255/40R23
Going from one to the other changes the overall height by -0.9 %. That is inside the 3% band, so it fits and the speedometer stays close.
Side by side
| Measure | 235/45R23 | 255/40R23 | Change |
|---|---|---|---|
| Overall height | 31.3" | 31.0" | -0.9 % |
| Sidewall | 4.16" | 4.02" | -3.5 % |
| Section width | 235 mm | 255 mm | +8.5 % |
| Circumference | 98.4" | 97.5" | -0.9 % |
| Revs per mile | 644 | 650 | +0.9 % |
| Axle height | 15.66" | 15.52" | -3.8 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 235/45R23. Fit 255/40R23 and the needle stops telling the truth.
| Needle says | You are doing | Out by |
|---|---|---|
| 30 mph | 29.7 mph | -0.3 |
| 50 mph | 49.5 mph | -0.5 |
| 70 mph | 69.3 mph | -0.7 |
| 90 mph | 89.2 mph | -0.8 |
The odometer drifts with it. Ten thousand miles on the dial are really 9,906 on the road, so it over-counts by 94. At a true 70 mph the first turns 751 times a minute, the second 758.