275/35R21 vs 265/40R21
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 | 275/35R21 | 265/40R21 | Change |
|---|---|---|---|
| Overall height | 28.6" | 29.3" | +2.7 % |
| Sidewall | 3.79" | 4.17" | +10.1 % |
| Section width | 275 mm | 265 mm | -3.6 % |
| Circumference | 89.8" | 92.2" | +2.7 % |
| Revs per mile | 706 | 687 | -2.7 % |
| Axle height | 14.29" | 14.67" | +9.8 mm |
| Approved rim width | 7.5–10" | 7.5–9.5" | overlap 7.5–9.5" |
Both are approved on a rim 7.5" 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 275/35R21. Fit 265/40R21 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 10,269 on the road, so it under-counts by 269. At a true 70 mph the first turns 823 times a minute, the second 802.