235/75R21 vs 255/65R21
Going from one to the other changes the overall height by -2.4 %. That is inside the 3% band, so it fits and the speedometer stays close.
Side by side
| Measure | 235/75R21 | 255/65R21 | Change |
|---|---|---|---|
| Overall height | 34.9" | 34.1" | -2.4 % |
| Sidewall | 6.94" | 6.53" | -6.0 % |
| Section width | 235 mm | 255 mm | +8.5 % |
| Circumference | 109.6" | 107.0" | -2.4 % |
| Revs per mile | 578 | 592 | +2.4 % |
| Axle height | 17.44" | 17.03" | -10.5 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 235/75R21. Fit 255/65R21 and the needle stops telling the truth.
| Needle says | You are doing | Out by |
|---|---|---|
| 30 mph | 29.3 mph | -0.7 |
| 50 mph | 48.8 mph | -1.2 |
| 70 mph | 68.3 mph | -1.7 |
| 90 mph | 87.9 mph | -2.1 |
The odometer drifts with it. Ten thousand miles on the dial are really 9,763 on the road, so it over-counts by 237. At a true 70 mph the first turns 675 times a minute, the second 691.