245/75R20 vs 265/65R20
Going from one to the other changes the overall height by -2.6 %. That is inside the 3% band, so it fits and the speedometer stays close.
Side by side
| Measure | 245/75R20 | 265/65R20 | Change |
|---|---|---|---|
| Overall height | 34.5" | 33.6" | -2.6 % |
| Sidewall | 7.23" | 6.78" | -6.3 % |
| Section width | 245 mm | 265 mm | +8.2 % |
| Circumference | 108.3" | 105.4" | -2.6 % |
| Revs per mile | 585 | 601 | +2.6 % |
| Axle height | 17.23" | 16.78" | -11.5 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 245/75R20. Fit 265/65R20 and the needle stops telling the truth.
| Needle says | You are doing | Out by |
|---|---|---|
| 30 mph | 29.2 mph | -0.8 |
| 50 mph | 48.7 mph | -1.3 |
| 70 mph | 68.2 mph | -1.8 |
| 90 mph | 87.6 mph | -2.4 |
The odometer drifts with it. Ten thousand miles on the dial are really 9,737 on the road, so it over-counts by 263. At a true 70 mph the first turns 683 times a minute, the second 701.