265/55R20 vs 245/55R20
Going from one to the other changes the overall height by -2.8 %. That is inside the 3% band, so it fits and the speedometer stays close.
Side by side
| Measure | 265/55R20 | 245/55R20 | Change |
|---|---|---|---|
| Overall height | 31.5" | 30.6" | -2.8 % |
| Sidewall | 5.74" | 5.31" | -7.5 % |
| Section width | 265 mm | 245 mm | -7.5 % |
| Circumference | 98.9" | 96.2" | -2.8 % |
| Revs per mile | 641 | 659 | +2.8 % |
| Axle height | 15.74" | 15.31" | -11.0 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 265/55R20. Fit 245/55R20 and the needle stops telling the truth.
| Needle says | You are doing | Out by |
|---|---|---|
| 30 mph | 29.2 mph | -0.8 |
| 50 mph | 48.6 mph | -1.4 |
| 70 mph | 68.1 mph | -1.9 |
| 90 mph | 87.5 mph | -2.5 |
The odometer drifts with it. Ten thousand miles on the dial are really 9,725 on the road, so it over-counts by 275. At a true 70 mph the first turns 748 times a minute, the second 769.