225/75R20 vs 235/75R20
Going from one to the other changes the overall height by +1.8 %. That is inside the 3% band, so it fits and the speedometer stays close.
Side by side
| Measure | 225/75R20 | 235/75R20 | Change |
|---|---|---|---|
| Overall height | 33.3" | 33.9" | +1.8 % |
| Sidewall | 6.64" | 6.94" | +4.4 % |
| Section width | 225 mm | 235 mm | +4.4 % |
| Circumference | 104.6" | 106.4" | +1.8 % |
| Revs per mile | 606 | 595 | -1.8 % |
| Axle height | 16.64" | 16.94" | +7.5 mm |
| Approved rim width | 6–8.5" | 6.5–8.5" | 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 225/75R20. Fit 235/75R20 and the needle stops telling the truth.
| Needle says | You are doing | Out by |
|---|---|---|
| 30 mph | 30.5 mph | +0.5 |
| 50 mph | 50.9 mph | +0.9 |
| 70 mph | 71.2 mph | +1.2 |
| 90 mph | 91.6 mph | +1.6 |
The odometer drifts with it. Ten thousand miles on the dial are really 10,177 on the road, so it under-counts by 177. At a true 70 mph the first turns 707 times a minute, the second 695.