235/45R20 vs 255/40R20
Going from one to the other changes the overall height by -1.0 %. That is inside the 3% band, so it fits and the speedometer stays close.
Side by side
| Measure | 235/45R20 | 255/40R20 | Change |
|---|---|---|---|
| Overall height | 28.3" | 28.0" | -1.0 % |
| Sidewall | 4.16" | 4.02" | -3.5 % |
| Section width | 235 mm | 255 mm | +8.5 % |
| Circumference | 89.0" | 88.1" | -1.0 % |
| Revs per mile | 712 | 719 | +1.0 % |
| Axle height | 14.16" | 14.02" | -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 235/45R20. 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 | 49.5 mph | -0.5 |
| 70 mph | 69.3 mph | -0.7 |
| 90 mph | 89.1 mph | -0.9 |
The odometer drifts with it. Ten thousand miles on the dial are really 9,896 on the road, so it over-counts by 104. At a true 70 mph the first turns 831 times a minute, the second 839.
What drivers reported
235/45R20 carries 2 complaints. Nobody has filed one naming 255/40R20.