265/65R21 vs 245/70R21
Going from one to the other changes the overall height by -0.2 %. That is inside the 3% band, so it fits and the speedometer stays close.
Side by side
| Measure | 265/65R21 | 245/70R21 | Change |
|---|---|---|---|
| Overall height | 34.6" | 34.5" | -0.2 % |
| Sidewall | 6.78" | 6.75" | -0.4 % |
| Section width | 265 mm | 245 mm | -7.5 % |
| Circumference | 108.6" | 108.4" | -0.2 % |
| Revs per mile | 584 | 585 | +0.2 % |
| Axle height | 17.28" | 17.25" | -0.8 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 265/65R21. Fit 245/70R21 and the needle stops telling the truth.
| Needle says | You are doing | Out by |
|---|---|---|
| 30 mph | 29.9 mph | -0.1 |
| 50 mph | 49.9 mph | -0.1 |
| 70 mph | 69.9 mph | -0.1 |
| 90 mph | 89.8 mph | -0.2 |
The odometer drifts with it. Ten thousand miles on the dial are really 9,983 on the road, so it over-counts by 17. At a true 70 mph the first turns 681 times a minute, the second 682.