255/35R20 vs 245/40R20
Going from one to the other changes the overall height by +2.5 %. That is inside the 3% band, so it fits and the speedometer stays close.
Side by side
| Measure | 255/35R20 | 245/40R20 | Change |
|---|---|---|---|
| Overall height | 27.0" | 27.7" | +2.5 % |
| Sidewall | 3.51" | 3.86" | +9.8 % |
| Section width | 255 mm | 245 mm | -3.9 % |
| Circumference | 84.9" | 87.1" | +2.5 % |
| Revs per mile | 746 | 728 | -2.5 % |
| Axle height | 13.51" | 13.86" | +8.8 mm |
| Approved rim width | 7–9.5" | 7–9" | 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 255/35R20. Fit 245/40R20 and the needle stops telling the truth.
| Needle says | You are doing | Out by |
|---|---|---|
| 30 mph | 30.8 mph | +0.8 |
| 50 mph | 51.3 mph | +1.3 |
| 70 mph | 71.8 mph | +1.8 |
| 90 mph | 92.3 mph | +2.3 |
The odometer drifts with it. Ten thousand miles on the dial are really 10,255 on the road, so it under-counts by 255. At a true 70 mph the first turns 871 times a minute, the second 849.
What drivers reported
255/35R20 carries 4 complaints. Nobody has filed one naming 245/40R20.