235/75R20 vs 245/70R20
Going from one to the other changes the overall height by -1.1 %. That is inside the 3% band, so it fits and the speedometer stays close.
Side by side
| Measure | 235/75R20 | 245/70R20 | Change |
|---|---|---|---|
| Overall height | 33.9" | 33.5" | -1.1 % |
| Sidewall | 6.94" | 6.75" | -2.7 % |
| Section width | 235 mm | 245 mm | +4.3 % |
| Circumference | 106.4" | 105.3" | -1.1 % |
| Revs per mile | 595 | 602 | +1.1 % |
| Axle height | 16.94" | 16.75" | -4.8 mm |
| Approved rim width | 6.5–8.5" | 7–9" | 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 235/75R20. Fit 245/70R20 and the needle stops telling the truth.
| Needle says | You are doing | Out by |
|---|---|---|
| 30 mph | 29.7 mph | -0.3 |
| 50 mph | 49.4 mph | -0.6 |
| 70 mph | 69.2 mph | -0.8 |
| 90 mph | 89.0 mph | -1.0 |
The odometer drifts with it. Ten thousand miles on the dial are really 9,890 on the road, so it over-counts by 110. At a true 70 mph the first turns 695 times a minute, the second 702.