265/30R21 vs 255/35R21
Going from one to the other changes the overall height by +2.8 %. That is inside the 3% band, so it fits and the speedometer stays close.
Side by side
| Measure | 265/30R21 | 255/35R21 | Change |
|---|---|---|---|
| Overall height | 27.3" | 28.0" | +2.8 % |
| Sidewall | 3.13" | 3.51" | +12.3 % |
| Section width | 265 mm | 255 mm | -3.8 % |
| Circumference | 85.6" | 88.1" | +2.8 % |
| Revs per mile | 740 | 720 | -2.8 % |
| Axle height | 13.63" | 14.01" | +9.8 mm |
| Approved rim width | 7.5–9.5" | 7–9.5" | 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 265/30R21. Fit 255/35R21 and the needle stops telling the truth.
| Needle says | You are doing | Out by |
|---|---|---|
| 30 mph | 30.8 mph | +0.8 |
| 50 mph | 51.4 mph | +1.4 |
| 70 mph | 72.0 mph | +2.0 |
| 90 mph | 92.5 mph | +2.5 |
The odometer drifts with it. Ten thousand miles on the dial are really 10,282 on the road, so it under-counts by 282. At a true 70 mph the first turns 863 times a minute, the second 840.