235/45R16 vs 255/45R16
Going from one to the other changes the overall height by +2.9 %. That is inside the 3% band, so it fits and the speedometer stays close.
Side by side
| Measure | 235/45R16 | 255/45R16 | Change |
|---|---|---|---|
| Overall height | 24.3" | 25.0" | +2.9 % |
| Sidewall | 4.16" | 4.52" | +8.5 % |
| Section width | 235 mm | 255 mm | +8.5 % |
| Circumference | 76.4" | 78.7" | +2.9 % |
| Revs per mile | 829 | 806 | -2.9 % |
| Axle height | 12.16" | 12.52" | +9.0 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 235/45R16. Fit 255/45R16 and the needle stops telling the truth.
| Needle says | You are doing | Out by |
|---|---|---|
| 30 mph | 30.9 mph | +0.9 |
| 50 mph | 51.5 mph | +1.5 |
| 70 mph | 72.0 mph | +2.0 |
| 90 mph | 92.6 mph | +2.6 |
The odometer drifts with it. Ten thousand miles on the dial are really 10,291 on the road, so it under-counts by 291. At a true 70 mph the first turns 967 times a minute, the second 940.