285/35R21 vs 265/40R21
Going from one to the other changes the overall height by +1.7 %. That is inside the 3% band, so it fits and the speedometer stays close.
Side by side
| Measure | 285/35R21 | 265/40R21 | Change |
|---|---|---|---|
| Overall height | 28.9" | 29.3" | +1.7 % |
| Sidewall | 3.93" | 4.17" | +6.3 % |
| Section width | 285 mm | 265 mm | -7.0 % |
| Circumference | 90.6" | 92.2" | +1.7 % |
| Revs per mile | 699 | 687 | -1.7 % |
| Axle height | 14.43" | 14.67" | +6.3 mm |
| Approved rim width | 8–10.5" | 7.5–9.5" | overlap 8–9.5" |
Both are approved on a rim 8" to 9.5" wide. If yours is in that band, this is a tire change and nothing else.
What the speedometer will say
Calibrated for 285/35R21. Fit 265/40R21 and the needle stops telling the truth.
| Needle says | You are doing | Out by |
|---|---|---|
| 30 mph | 30.5 mph | +0.5 |
| 50 mph | 50.9 mph | +0.9 |
| 70 mph | 71.2 mph | +1.2 |
| 90 mph | 91.5 mph | +1.5 |
The odometer drifts with it. Ten thousand miles on the dial are really 10,171 on the road, so it under-counts by 171. At a true 70 mph the first turns 815 times a minute, the second 802.