265/35R23 vs 255/40R23
Going from one to the other changes the overall height by +2.4 %. That is inside the 3% band, so it fits and the speedometer stays close.
Side by side
| Measure | 265/35R23 | 255/40R23 | Change |
|---|---|---|---|
| Overall height | 30.3" | 31.0" | +2.4 % |
| Sidewall | 3.65" | 4.02" | +10.0 % |
| Section width | 265 mm | 255 mm | -3.8 % |
| Circumference | 95.2" | 97.5" | +2.4 % |
| Revs per mile | 666 | 650 | -2.4 % |
| Axle height | 15.15" | 15.52" | +9.3 mm |
| Approved rim width | 7.5–9.5" | 7–9.5" | overlap 7.5–9.5" |
Both are approved on a rim 7.5" to 9.5" wide. If yours is in that band, this is a tire change and nothing else.
What the speedometer will say
Calibrated for 265/35R23. Fit 255/40R23 and the needle stops telling the truth.
| Needle says | You are doing | Out by |
|---|---|---|
| 30 mph | 30.7 mph | +0.7 |
| 50 mph | 51.2 mph | +1.2 |
| 70 mph | 71.7 mph | +1.7 |
| 90 mph | 92.2 mph | +2.2 |
The odometer drifts with it. Ten thousand miles on the dial are really 10,240 on the road, so it under-counts by 240. At a true 70 mph the first turns 776 times a minute, the second 758.