265/75R20 vs 245/80R20
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 | 265/75R20 | 245/80R20 | Change |
|---|---|---|---|
| Overall height | 35.6" | 35.4" | -0.6 % |
| Sidewall | 7.82" | 7.72" | -1.4 % |
| Section width | 265 mm | 245 mm | -7.5 % |
| Circumference | 112.0" | 111.3" | -0.6 % |
| Revs per mile | 566 | 569 | +0.6 % |
| Axle height | 17.82" | 17.72" | -2.8 mm |
| Approved rim width | 7.5–9.5" | 7–9" | 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 265/75R20. Fit 245/80R20 and the needle stops telling the truth.
| Needle says | You are doing | Out by |
|---|---|---|
| 30 mph | 29.8 mph | -0.2 |
| 50 mph | 49.7 mph | -0.3 |
| 70 mph | 69.6 mph | -0.4 |
| 90 mph | 89.5 mph | -0.5 |
The odometer drifts with it. Ten thousand miles on the dial are really 9,939 on the road, so it over-counts by 61. At a true 70 mph the first turns 660 times a minute, the second 664.