295/40R21 vs 285/45R21
Going from one to the other changes the overall height by +2.7 %. That is inside the 3% band, so it fits and the speedometer stays close.
Side by side
| Measure | 295/40R21 | 285/45R21 | Change |
|---|---|---|---|
| Overall height | 30.3" | 31.1" | +2.7 % |
| Sidewall | 4.65" | 5.05" | +8.7 % |
| Section width | 295 mm | 285 mm | -3.4 % |
| Circumference | 95.2" | 97.7" | +2.7 % |
| Revs per mile | 666 | 649 | -2.7 % |
| Axle height | 15.15" | 15.55" | +10.3 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 295/40R21. Fit 285/45R21 and the needle stops telling the truth.
| Needle says | You are doing | Out by |
|---|---|---|
| 30 mph | 30.8 mph | +0.8 |
| 50 mph | 51.3 mph | +1.3 |
| 70 mph | 71.9 mph | +1.9 |
| 90 mph | 92.4 mph | +2.4 |
The odometer drifts with it. Ten thousand miles on the dial are really 10,266 on the road, so it under-counts by 266. At a true 70 mph the first turns 777 times a minute, the second 757.
What drivers reported
295/40R21 carries 2 complaints. Nobody has filed one naming 285/45R21.