275/30R21 vs 265/35R21
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 | 275/30R21 | 265/35R21 | Change |
|---|---|---|---|
| Overall height | 27.5" | 28.3" | +2.9 % |
| Sidewall | 3.25" | 3.65" | +12.4 % |
| Section width | 275 mm | 265 mm | -3.6 % |
| Circumference | 86.4" | 88.9" | +2.9 % |
| Revs per mile | 733 | 713 | -2.9 % |
| Axle height | 13.75" | 14.15" | +10.3 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 275/30R21. Fit 265/35R21 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 | 72.1 mph | +2.1 |
| 90 mph | 92.6 mph | +2.6 |
The odometer drifts with it. Ten thousand miles on the dial are really 10,294 on the road, so it under-counts by 294. At a true 70 mph the first turns 856 times a minute, the second 831.