235/35R20 vs 225/40R20
Going from one to the other changes the overall height by +2.3 %. That is inside the 3% band, so it fits and the speedometer stays close.
Side by side
| Measure | 235/35R20 | 225/40R20 | Change |
|---|---|---|---|
| Overall height | 26.5" | 27.1" | +2.3 % |
| Sidewall | 3.24" | 3.54" | +9.4 % |
| Section width | 235 mm | 225 mm | -4.3 % |
| Circumference | 83.2" | 85.1" | +2.3 % |
| Revs per mile | 762 | 745 | -2.3 % |
| Axle height | 13.24" | 13.54" | +7.8 mm |
| Approved rim width | 6.5–8.5" | 6–8.5" | overlap 6.5–8.5" |
Both are approved on a rim 6.5" 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 235/35R20. Fit 225/40R20 and the needle stops telling the truth.
| Needle says | You are doing | Out by |
|---|---|---|
| 30 mph | 30.7 mph | +0.7 |
| 50 mph | 51.2 mph | +1.2 |
| 70 mph | 71.6 mph | +1.6 |
| 90 mph | 92.1 mph | +2.1 |
The odometer drifts with it. Ten thousand miles on the dial are really 10,230 on the road, so it under-counts by 230. At a true 70 mph the first turns 889 times a minute, the second 869.