295/35R21 vs 285/40R21
Going from one to the other changes the overall height by +2.9 %. That is inside the 3% band, so it fits and the speedometer stays close.
Side by side
| Measure | 295/35R21 | 285/40R21 | Change |
|---|---|---|---|
| Overall height | 29.1" | 30.0" | +2.9 % |
| Sidewall | 4.06" | 4.49" | +10.4 % |
| Section width | 295 mm | 285 mm | -3.4 % |
| Circumference | 91.5" | 94.2" | +2.9 % |
| Revs per mile | 692 | 673 | -2.9 % |
| Axle height | 14.56" | 14.99" | +10.8 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 295/35R21. Fit 285/40R21 and the needle stops telling the truth.
| Needle says | You are doing | Out by |
|---|---|---|
| 30 mph | 30.9 mph | +0.9 |
| 50 mph | 51.5 mph | +1.5 |
| 70 mph | 72.0 mph | +2.0 |
| 90 mph | 92.6 mph | +2.6 |
The odometer drifts with it. Ten thousand miles on the dial are really 10,291 on the road, so it under-counts by 291. At a true 70 mph the first turns 808 times a minute, the second 785.
What drivers reported
295/35R21 carries 1 complaints. Nobody has filed one naming 285/40R21.