245/40R20 vs 265/35R20
Going from one to the other changes the overall height by -1.5 %. That is inside the 3% band, so it fits and the speedometer stays close.
Side by side
| Measure | 245/40R20 | 265/35R20 | Change |
|---|---|---|---|
| Overall height | 27.7" | 27.3" | -1.5 % |
| Sidewall | 3.86" | 3.65" | -5.4 % |
| Section width | 245 mm | 265 mm | +8.2 % |
| Circumference | 87.1" | 85.8" | -1.5 % |
| Revs per mile | 728 | 739 | +1.5 % |
| Axle height | 13.86" | 13.65" | -5.3 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 245/40R20. Fit 265/35R20 and the needle stops telling the truth.
| Needle says | You are doing | Out by |
|---|---|---|
| 30 mph | 29.6 mph | -0.4 |
| 50 mph | 49.3 mph | -0.7 |
| 70 mph | 69.0 mph | -1.0 |
| 90 mph | 88.7 mph | -1.3 |
The odometer drifts with it. Ten thousand miles on the dial are really 9,851 on the road, so it over-counts by 149. At a true 70 mph the first turns 849 times a minute, the second 862.