265/75R21 vs 265/70R21
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 | 265/75R21 | 265/70R21 | Change |
|---|---|---|---|
| Overall height | 36.6" | 35.6" | -2.8 % |
| Sidewall | 7.82" | 7.30" | -6.7 % |
| Section width | 265 mm | 265 mm | 0.0 % |
| Circumference | 115.1" | 111.9" | -2.8 % |
| Revs per mile | 550 | 566 | +2.8 % |
| Axle height | 18.32" | 17.80" | -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 265/75R21. Fit 265/70R21 and the needle stops telling the truth.
| Needle says | You are doing | Out by |
|---|---|---|
| 30 mph | 29.1 mph | -0.9 |
| 50 mph | 48.6 mph | -1.4 |
| 70 mph | 68.0 mph | -2.0 |
| 90 mph | 87.4 mph | -2.6 |
The odometer drifts with it. Ten thousand miles on the dial are really 9,715 on the road, so it over-counts by 285. At a true 70 mph the first turns 642 times a minute, the second 661.