245/40R20 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 | 245/40R20 | 255/40R20 | Change |
|---|---|---|---|
| Overall height | 27.7" | 28.0" | +1.1 % |
| Sidewall | 3.86" | 4.02" | +4.1 % |
| Section width | 245 mm | 255 mm | +4.1 % |
| Circumference | 87.1" | 88.1" | +1.1 % |
| Revs per mile | 728 | 719 | -1.1 % |
| Axle height | 13.86" | 14.02" | +4.0 mm |
| Approved rim width | 7–9" | 7–9.5" | overlap 7–9" |
Both are approved on a rim 7" to 9" wide. If yours is in that band, this is a tire change and nothing else.
What the speedometer will say
Calibrated for 245/40R20. Fit 255/40R20 and the needle stops telling the truth.
| Needle says | You are doing | Out by |
|---|---|---|
| 30 mph | 30.3 mph | +0.3 |
| 50 mph | 50.6 mph | +0.6 |
| 70 mph | 70.8 mph | +0.8 |
| 90 mph | 91.0 mph | +1.0 |
The odometer drifts with it. Ten thousand miles on the dial are really 10,114 on the road, so it under-counts by 114. At a true 70 mph the first turns 849 times a minute, the second 839.