255/75R17 vs 235/80R17
Going from one to the other changes the overall height by -0.8 %. That is inside the 3% band, so it fits and the speedometer stays close.
Side by side
| Measure | 255/75R17 | 235/80R17 | Change |
|---|---|---|---|
| Overall height | 32.1" | 31.8" | -0.8 % |
| Sidewall | 7.53" | 7.40" | -1.7 % |
| Section width | 255 mm | 235 mm | -7.8 % |
| Circumference | 100.7" | 99.9" | -0.8 % |
| Revs per mile | 629 | 634 | +0.8 % |
| Axle height | 16.03" | 15.90" | -3.3 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 255/75R17. Fit 235/80R17 and the needle stops telling the truth.
| Needle says | You are doing | Out by |
|---|---|---|
| 30 mph | 29.8 mph | -0.2 |
| 50 mph | 49.6 mph | -0.4 |
| 70 mph | 69.4 mph | -0.6 |
| 90 mph | 89.3 mph | -0.7 |
The odometer drifts with it. Ten thousand miles on the dial are really 9,920 on the road, so it over-counts by 80. At a true 70 mph the first turns 734 times a minute, the second 740.