265/65R21 vs 245/65R21
Going from one to the other changes the overall height by -3.0 %. That is inside the 3% band, so it fits and the speedometer stays close.
Side by side
| Measure | 265/65R21 | 245/65R21 | Change |
|---|---|---|---|
| Overall height | 34.6" | 33.5" | -3.0 % |
| Sidewall | 6.78" | 6.27" | -7.5 % |
| Section width | 265 mm | 245 mm | -7.5 % |
| Circumference | 108.6" | 105.4" | -3.0 % |
| Revs per mile | 584 | 601 | +3.0 % |
| Axle height | 17.28" | 16.77" | -13.0 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 265/65R21. Fit 245/65R21 and the needle stops telling the truth.
| Needle says | You are doing | Out by |
|---|---|---|
| 30 mph | 29.1 mph | -0.9 |
| 50 mph | 48.5 mph | -1.5 |
| 70 mph | 67.9 mph | -2.1 |
| 90 mph | 87.3 mph | -2.7 |
The odometer drifts with it. Ten thousand miles on the dial are really 9,704 on the road, so it over-counts by 296. At a true 70 mph the first turns 681 times a minute, the second 702.