255/40R16 vs 235/45R16
Going from one to the other changes the overall height by +1.2 %. That is inside the 3% band, so it fits and the speedometer stays close.
Side by side
| Measure | 255/40R16 | 235/45R16 | Change |
|---|---|---|---|
| Overall height | 24.0" | 24.3" | +1.2 % |
| Sidewall | 4.02" | 4.16" | +3.7 % |
| Section width | 255 mm | 235 mm | -7.8 % |
| Circumference | 75.5" | 76.4" | +1.2 % |
| Revs per mile | 839 | 829 | -1.2 % |
| Axle height | 12.02" | 12.16" | +3.8 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 255/40R16. Fit 235/45R16 and the needle stops telling the truth.
| Needle says | You are doing | Out by |
|---|---|---|
| 30 mph | 30.4 mph | +0.4 |
| 50 mph | 50.6 mph | +0.6 |
| 70 mph | 70.9 mph | +0.9 |
| 90 mph | 91.1 mph | +1.1 |
The odometer drifts with it. Ten thousand miles on the dial are really 10,123 on the road, so it under-counts by 123. At a true 70 mph the first turns 979 times a minute, the second 967.