265/70R21 vs 265/65R21
Going from one to the other changes the overall height by -2.9 %. That is inside the 3% band, so it fits and the speedometer stays close.
Side by side
| Measure | 265/70R21 | 265/65R21 | Change |
|---|---|---|---|
| Overall height | 35.6" | 34.6" | -2.9 % |
| Sidewall | 7.30" | 6.78" | -7.1 % |
| Section width | 265 mm | 265 mm | 0.0 % |
| Circumference | 111.9" | 108.6" | -2.9 % |
| Revs per mile | 566 | 584 | +2.9 % |
| Axle height | 17.80" | 17.28" | -13.3 mm |
| Approved rim width | 7.5–9.5" | 7.5–9.5" | overlap 7.5–9.5" |
Both are approved on a rim 7.5" to 9.5" wide. If yours is in that band, this is a tire change and nothing else.
What the speedometer will say
Calibrated for 265/70R21. Fit 265/65R21 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 | 87.4 mph | -2.6 |
The odometer drifts with it. Ten thousand miles on the dial are really 9,707 on the road, so it over-counts by 293. At a true 70 mph the first turns 661 times a minute, the second 681.