235/35R21 vs 255/35R21
Going from one to the other changes the overall height by +2.0 %. That is inside the 3% band, so it fits and the speedometer stays close.
Side by side
| Measure | 235/35R21 | 255/35R21 | Change |
|---|---|---|---|
| Overall height | 27.5" | 28.0" | +2.0 % |
| Sidewall | 3.24" | 3.51" | +8.5 % |
| Section width | 235 mm | 255 mm | +8.5 % |
| Circumference | 86.3" | 88.1" | +2.0 % |
| Revs per mile | 734 | 720 | -2.0 % |
| Axle height | 13.74" | 14.01" | +7.0 mm |
| Approved rim width | 6.5–8.5" | 7–9.5" | overlap 7–8.5" |
Both are approved on a rim 7" to 8.5" wide. If yours is in that band, this is a tire change and nothing else.
What the speedometer will say
Calibrated for 235/35R21. Fit 255/35R21 and the needle stops telling the truth.
| Needle says | You are doing | Out by |
|---|---|---|
| 30 mph | 30.6 mph | +0.6 |
| 50 mph | 51.0 mph | +1.0 |
| 70 mph | 71.4 mph | +1.4 |
| 90 mph | 91.8 mph | +1.8 |
The odometer drifts with it. Ten thousand miles on the dial are really 10,201 on the road, so it under-counts by 201. At a true 70 mph the first turns 856 times a minute, the second 840.