265/40R20 vs 255/40R20
Going from one to the other changes the overall height by -1.1 %. That is inside the 3% band, so it fits and the speedometer stays close.
Side by side
| Measure | 265/40R20 | 255/40R20 | Change |
|---|---|---|---|
| Overall height | 28.3" | 28.0" | -1.1 % |
| Sidewall | 4.17" | 4.02" | -3.8 % |
| Section width | 265 mm | 255 mm | -3.8 % |
| Circumference | 89.1" | 88.1" | -1.1 % |
| Revs per mile | 711 | 719 | +1.1 % |
| Axle height | 14.17" | 14.02" | -4.0 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 265/40R20. Fit 255/40R20 and the needle stops telling the truth.
| Needle says | You are doing | Out by |
|---|---|---|
| 30 mph | 29.7 mph | -0.3 |
| 50 mph | 49.4 mph | -0.6 |
| 70 mph | 69.2 mph | -0.8 |
| 90 mph | 89.0 mph | -1.0 |
The odometer drifts with it. Ten thousand miles on the dial are really 9,889 on the road, so it over-counts by 111. At a true 70 mph the first turns 830 times a minute, the second 839.