255/65R16 vs 235/75R16
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 | 255/65R16 | 235/75R16 | Change |
|---|---|---|---|
| Overall height | 29.1" | 29.9" | +2.8 % |
| Sidewall | 6.53" | 6.94" | +6.3 % |
| Section width | 255 mm | 235 mm | -7.8 % |
| Circumference | 91.3" | 93.9" | +2.8 % |
| Revs per mile | 694 | 675 | -2.8 % |
| Axle height | 14.53" | 14.94" | +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 255/65R16. Fit 235/75R16 and the needle stops telling the truth.
| Needle says | You are doing | Out by |
|---|---|---|
| 30 mph | 30.9 mph | +0.9 |
| 50 mph | 51.4 mph | +1.4 |
| 70 mph | 72.0 mph | +2.0 |
| 90 mph | 92.6 mph | +2.6 |
The odometer drifts with it. Ten thousand miles on the dial are really 10,285 on the road, so it under-counts by 285. At a true 70 mph the first turns 810 times a minute, the second 788.