245/80R20 vs 265/75R20
Going from one to the other changes the overall height by +0.6 %. That is inside the 3% band, so it fits and the speedometer stays close.
Side by side
| Measure | 245/80R20 | 265/75R20 | Change |
|---|---|---|---|
| Overall height | 35.4" | 35.6" | +0.6 % |
| Sidewall | 7.72" | 7.82" | +1.4 % |
| Section width | 245 mm | 265 mm | +8.2 % |
| Circumference | 111.3" | 112.0" | +0.6 % |
| Revs per mile | 569 | 566 | -0.6 % |
| Axle height | 17.72" | 17.82" | +2.8 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 245/80R20. Fit 265/75R20 and the needle stops telling the truth.
| Needle says | You are doing | Out by |
|---|---|---|
| 30 mph | 30.2 mph | +0.2 |
| 50 mph | 50.3 mph | +0.3 |
| 70 mph | 70.4 mph | +0.4 |
| 90 mph | 90.6 mph | +0.6 |
The odometer drifts with it. Ten thousand miles on the dial are really 10,061 on the road, so it under-counts by 61. At a true 70 mph the first turns 664 times a minute, the second 660.