295/75R21 vs 285/75R21
Going from one to the other changes the overall height by -1.5 %. That is inside the 3% band, so it fits and the speedometer stays close.
Side by side
| Measure | 295/75R21 | 285/75R21 | Change |
|---|---|---|---|
| Overall height | 38.4" | 37.8" | -1.5 % |
| Sidewall | 8.71" | 8.42" | -3.4 % |
| Section width | 295 mm | 285 mm | -3.4 % |
| Circumference | 120.7" | 118.8" | -1.5 % |
| Revs per mile | 525 | 533 | +1.5 % |
| Axle height | 19.21" | 18.92" | -7.5 mm |
| Approved rim width | 8.5–11" | 8–10.5" | overlap 8.5–10.5" |
Both are approved on a rim 8.5" 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 295/75R21. Fit 285/75R21 and the needle stops telling the truth.
| Needle says | You are doing | Out by |
|---|---|---|
| 30 mph | 29.5 mph | -0.5 |
| 50 mph | 49.2 mph | -0.8 |
| 70 mph | 68.9 mph | -1.1 |
| 90 mph | 88.6 mph | -1.4 |
The odometer drifts with it. Ten thousand miles on the dial are really 9,846 on the road, so it over-counts by 154. At a true 70 mph the first turns 612 times a minute, the second 622.