295/45R21 vs 285/45R21
Going from one to the other changes the overall height by -1.1 %. That is inside the 3% band, so it fits and the speedometer stays close.
Side by side
| Measure | 295/45R21 | 285/45R21 | Change |
|---|---|---|---|
| Overall height | 31.5" | 31.1" | -1.1 % |
| Sidewall | 5.23" | 5.05" | -3.4 % |
| Section width | 295 mm | 285 mm | -3.4 % |
| Circumference | 98.8" | 97.7" | -1.1 % |
| Revs per mile | 641 | 649 | +1.1 % |
| Axle height | 15.73" | 15.55" | -4.5 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 295/45R21. Fit 285/45R21 and the needle stops telling the truth.
| Needle says | You are doing | Out by |
|---|---|---|
| 30 mph | 29.7 mph | -0.3 |
| 50 mph | 49.4 mph | -0.6 |
| 70 mph | 69.2 mph | -0.8 |
| 90 mph | 89.0 mph | -1.0 |
The odometer drifts with it. Ten thousand miles on the dial are really 9,887 on the road, so it over-counts by 113. At a true 70 mph the first turns 748 times a minute, the second 757.