245/85R20 vs 235/85R20
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 | 245/85R20 | 235/85R20 | Change |
|---|---|---|---|
| Overall height | 36.4" | 35.7" | -1.8 % |
| Sidewall | 8.20" | 7.86" | -4.1 % |
| Section width | 245 mm | 235 mm | -4.1 % |
| Circumference | 114.3" | 112.2" | -1.8 % |
| Revs per mile | 554 | 564 | +1.8 % |
| Axle height | 18.20" | 17.86" | -8.5 mm |
| Approved rim width | 7–9" | 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 245/85R20. Fit 235/85R20 and the needle stops telling the truth.
| Needle says | You are doing | Out by |
|---|---|---|
| 30 mph | 29.4 mph | -0.6 |
| 50 mph | 49.1 mph | -0.9 |
| 70 mph | 68.7 mph | -1.3 |
| 90 mph | 88.3 mph | -1.7 |
The odometer drifts with it. Ten thousand miles on the dial are really 9,816 on the road, so it over-counts by 184. At a true 70 mph the first turns 646 times a minute, the second 659.