255/60R23 vs 235/70R23
Going from one to the other changes the overall height by +2.6 %. That is inside the 3% band, so it fits and the speedometer stays close.
Side by side
| Measure | 255/60R23 | 235/70R23 | Change |
|---|---|---|---|
| Overall height | 35.0" | 36.0" | +2.6 % |
| Sidewall | 6.02" | 6.48" | +7.5 % |
| Section width | 255 mm | 235 mm | -7.8 % |
| Circumference | 110.1" | 112.9" | +2.6 % |
| Revs per mile | 575 | 561 | -2.6 % |
| Axle height | 17.52" | 17.98" | +11.5 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 255/60R23. Fit 235/70R23 and the needle stops telling the truth.
| Needle says | You are doing | Out by |
|---|---|---|
| 30 mph | 30.8 mph | +0.8 |
| 50 mph | 51.3 mph | +1.3 |
| 70 mph | 71.8 mph | +1.8 |
| 90 mph | 92.3 mph | +2.3 |
The odometer drifts with it. Ten thousand miles on the dial are really 10,258 on the road, so it under-counts by 258. At a true 70 mph the first turns 671 times a minute, the second 654.