235/80R17 vs 255/70R17
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 | 235/80R17 | 255/70R17 | Change |
|---|---|---|---|
| Overall height | 31.8" | 31.1" | -2.4 % |
| Sidewall | 7.40" | 7.03" | -5.1 % |
| Section width | 235 mm | 255 mm | +8.5 % |
| Circumference | 99.9" | 97.6" | -2.4 % |
| Revs per mile | 634 | 649 | +2.4 % |
| Axle height | 15.90" | 15.53" | -9.5 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 235/80R17. Fit 255/70R17 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 | 68.4 mph | -1.6 |
| 90 mph | 87.9 mph | -2.1 |
The odometer drifts with it. Ten thousand miles on the dial are really 9,765 on the road, so it over-counts by 235. At a true 70 mph the first turns 740 times a minute, the second 758.