235/70R23 vs 255/60R23
Going from one to the other changes the overall height by -2.5 %. That is inside the 3% band, so it fits and the speedometer stays close.
Side by side
| Measure | 235/70R23 | 255/60R23 | Change |
|---|---|---|---|
| Overall height | 36.0" | 35.0" | -2.5 % |
| Sidewall | 6.48" | 6.02" | -7.0 % |
| Section width | 235 mm | 255 mm | +8.5 % |
| Circumference | 112.9" | 110.1" | -2.5 % |
| Revs per mile | 561 | 575 | +2.5 % |
| Axle height | 17.98" | 17.52" | -11.5 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 235/70R23. Fit 255/60R23 and the needle stops telling the truth.
| Needle says | You are doing | Out by |
|---|---|---|
| 30 mph | 29.2 mph | -0.8 |
| 50 mph | 48.7 mph | -1.3 |
| 70 mph | 68.2 mph | -1.8 |
| 90 mph | 87.7 mph | -2.3 |
The odometer drifts with it. Ten thousand miles on the dial are really 9,748 on the road, so it over-counts by 252. At a true 70 mph the first turns 654 times a minute, the second 671.