285/75R21 vs 285/70R21
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 | 285/75R21 | 285/70R21 | Change |
|---|---|---|---|
| Overall height | 37.8" | 36.7" | -3.0 % |
| Sidewall | 8.42" | 7.85" | -6.7 % |
| Section width | 285 mm | 285 mm | 0.0 % |
| Circumference | 118.8" | 115.3" | -3.0 % |
| Revs per mile | 533 | 549 | +3.0 % |
| Axle height | 18.92" | 18.35" | -14.3 mm |
| Approved rim width | 8–10.5" | 8–10.5" | overlap 8–10.5" |
Both are approved on a rim 8" to 10.5" wide. If yours is in that band, this is a tire change and nothing else.
What the speedometer will say
Calibrated for 285/75R21. Fit 285/70R21 and the needle stops telling the truth.
| Needle says | You are doing | Out by |
|---|---|---|
| 30 mph | 29.1 mph | -0.9 |
| 50 mph | 48.5 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 9,703 on the road, so it over-counts by 297. At a true 70 mph the first turns 622 times a minute, the second 641.