235/75R17.5 vs 245/70R17.5
Going from one to the other changes the overall height by -1.2 %. That is inside the 3% band, so it fits and the speedometer stays close.
Side by side
| Measure | 235/75R17.5 | 245/70R17.5 | Change |
|---|---|---|---|
| Overall height | 31.4" | 31.0" | -1.2 % |
| Sidewall | 6.94" | 6.75" | -2.7 % |
| Section width | 235 mm | 245 mm | +4.3 % |
| Circumference | 98.6" | 97.4" | -1.2 % |
| Revs per mile | 643 | 651 | +1.2 % |
| Axle height | 15.69" | 15.50" | -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 235/75R17.5. Fit 245/70R17.5 and the needle stops telling the truth.
| Needle says | You are doing | Out by |
|---|---|---|
| 30 mph | 29.6 mph | -0.4 |
| 50 mph | 49.4 mph | -0.6 |
| 70 mph | 69.2 mph | -0.8 |
| 90 mph | 88.9 mph | -1.1 |
The odometer drifts with it. Ten thousand miles on the dial are really 9,881 on the road, so it over-counts by 119. At a true 70 mph the first turns 750 times a minute, the second 759.