255/35R20 vs 235/40R20
Going from one to the other changes the overall height by +1.4 %. That is inside the 3% band, so it fits and the speedometer stays close.
Side by side
| Measure | 255/35R20 | 235/40R20 | Change |
|---|---|---|---|
| Overall height | 27.0" | 27.4" | +1.4 % |
| Sidewall | 3.51" | 3.70" | +5.3 % |
| Section width | 255 mm | 235 mm | -7.8 % |
| Circumference | 84.9" | 86.1" | +1.4 % |
| Revs per mile | 746 | 736 | -1.4 % |
| Axle height | 13.51" | 13.70" | +4.8 mm |
| Approved rim width | 7–9.5" | 6.5–8.5" | 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 255/35R20. Fit 235/40R20 and the needle stops telling the truth.
| Needle says | You are doing | Out by |
|---|---|---|
| 30 mph | 30.4 mph | +0.4 |
| 50 mph | 50.7 mph | +0.7 |
| 70 mph | 71.0 mph | +1.0 |
| 90 mph | 91.2 mph | +1.2 |
The odometer drifts with it. Ten thousand miles on the dial are really 10,138 on the road, so it under-counts by 138. At a true 70 mph the first turns 871 times a minute, the second 859.
What drivers reported
255/35R20 carries 4 complaints. Nobody has filed one naming 235/40R20.