235/75R17 vs 245/70R17
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 | 245/70R17 | Change |
|---|---|---|---|
| Overall height | 30.9" | 30.5" | -1.2 % |
| Sidewall | 6.94" | 6.75" | -2.7 % |
| Section width | 235 mm | 245 mm | +4.3 % |
| Circumference | 97.0" | 95.8" | -1.2 % |
| Revs per mile | 653 | 661 | +1.2 % |
| Axle height | 15.44" | 15.25" | -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. Fit 245/70R17 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 9,879 on the road, so it over-counts by 121. At a true 70 mph the first turns 762 times a minute, the second 771.