255/65R21 vs 235/75R21
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 | 255/65R21 | 235/75R21 | Change |
|---|---|---|---|
| Overall height | 34.1" | 34.9" | +2.4 % |
| Sidewall | 6.53" | 6.94" | +6.3 % |
| Section width | 255 mm | 235 mm | -7.8 % |
| Circumference | 107.0" | 109.6" | +2.4 % |
| Revs per mile | 592 | 578 | -2.4 % |
| Axle height | 17.03" | 17.44" | +10.5 mm |
| Approved rim width | 7–9.5" | 6.5–8.5" | 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 255/65R21. Fit 235/75R21 and the needle stops telling the truth.
| Needle says | You are doing | Out by |
|---|---|---|
| 30 mph | 30.7 mph | +0.7 |
| 50 mph | 51.2 mph | +1.2 |
| 70 mph | 71.7 mph | +1.7 |
| 90 mph | 92.2 mph | +2.2 |
The odometer drifts with it. Ten thousand miles on the dial are really 10,243 on the road, so it under-counts by 243. At a true 70 mph the first turns 691 times a minute, the second 675.