245/40R22 vs 255/40R22
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 | 245/40R22 | 255/40R22 | Change |
|---|---|---|---|
| Overall height | 29.7" | 30.0" | +1.1 % |
| Sidewall | 3.86" | 4.02" | +4.1 % |
| Section width | 245 mm | 255 mm | +4.1 % |
| Circumference | 93.4" | 94.3" | +1.1 % |
| Revs per mile | 679 | 672 | -1.1 % |
| Axle height | 14.86" | 15.02" | +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 245/40R22. Fit 255/40R22 and the needle stops telling the truth.
| Needle says | You are doing | Out by |
|---|---|---|
| 30 mph | 30.3 mph | +0.3 |
| 50 mph | 50.5 mph | +0.5 |
| 70 mph | 70.7 mph | +0.7 |
| 90 mph | 91.0 mph | +1.0 |
The odometer drifts with it. Ten thousand miles on the dial are really 10,106 on the road, so it under-counts by 106. At a true 70 mph the first turns 792 times a minute, the second 783.