255/70R17 vs 235/80R17
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 | 255/70R17 | 235/80R17 | Change |
|---|---|---|---|
| Overall height | 31.1" | 31.8" | +2.4 % |
| Sidewall | 7.03" | 7.40" | +5.3 % |
| Section width | 255 mm | 235 mm | -7.8 % |
| Circumference | 97.6" | 99.9" | +2.4 % |
| Revs per mile | 649 | 634 | -2.4 % |
| Axle height | 15.53" | 15.90" | +9.5 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 255/70R17. Fit 235/80R17 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 10,241 on the road, so it under-counts by 241. At a true 70 mph the first turns 758 times a minute, the second 740.