245/75R17.5 vs 265/70R17.5
Going from one to the other changes the overall height by +0.4 %. That is inside the 3% band, so it fits and the speedometer stays close.
Side by side
| Measure | 245/75R17.5 | 265/70R17.5 | Change |
|---|---|---|---|
| Overall height | 32.0" | 32.1" | +0.4 % |
| Sidewall | 7.23" | 7.30" | +1.0 % |
| Section width | 245 mm | 265 mm | +8.2 % |
| Circumference | 100.4" | 100.9" | +0.4 % |
| Revs per mile | 631 | 628 | -0.4 % |
| Axle height | 15.98" | 16.05" | +1.8 mm |
| Approved rim width | 7–9" | 7.5–9.5" | overlap 7.5–9" |
Both are approved on a rim 7.5" to 9" wide. If yours is in that band, this is a tire change and nothing else.
What the speedometer will say
Calibrated for 245/75R17.5. Fit 265/70R17.5 and the needle stops telling the truth.
| Needle says | You are doing | Out by |
|---|---|---|
| 30 mph | 30.1 mph | +0.1 |
| 50 mph | 50.2 mph | +0.2 |
| 70 mph | 70.3 mph | +0.3 |
| 90 mph | 90.4 mph | +0.4 |
The odometer drifts with it. Ten thousand miles on the dial are really 10,043 on the road, so it under-counts by 43. At a true 70 mph the first turns 736 times a minute, the second 733.