295/30R21 vs 285/30R21
Going from one to the other changes the overall height by -0.8 %. That is inside the 3% band, so it fits and the speedometer stays close.
Side by side
| Measure | 295/30R21 | 285/30R21 | Change |
|---|---|---|---|
| Overall height | 28.0" | 27.7" | -0.8 % |
| Sidewall | 3.48" | 3.37" | -3.4 % |
| Section width | 295 mm | 285 mm | -3.4 % |
| Circumference | 87.9" | 87.1" | -0.8 % |
| Revs per mile | 721 | 727 | +0.8 % |
| Axle height | 13.98" | 13.87" | -3.0 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 295/30R21. Fit 285/30R21 and the needle stops telling the truth.
| Needle says | You are doing | Out by |
|---|---|---|
| 30 mph | 29.7 mph | -0.3 |
| 50 mph | 49.6 mph | -0.4 |
| 70 mph | 69.4 mph | -0.6 |
| 90 mph | 89.2 mph | -0.8 |
The odometer drifts with it. Ten thousand miles on the dial are really 9,916 on the road, so it over-counts by 84. At a true 70 mph the first turns 841 times a minute, the second 848.