245/40R23 vs 235/45R23
Going from one to the other changes the overall height by +2.0 %. That is inside the 3% band, so it fits and the speedometer stays close.
Side by side
| Measure | 245/40R23 | 235/45R23 | Change |
|---|---|---|---|
| Overall height | 30.7" | 31.3" | +2.0 % |
| Sidewall | 3.86" | 4.16" | +7.9 % |
| Section width | 245 mm | 235 mm | -4.1 % |
| Circumference | 96.5" | 98.4" | +2.0 % |
| Revs per mile | 657 | 644 | -2.0 % |
| Axle height | 15.36" | 15.66" | +7.8 mm |
| Approved rim width | 7–9" | 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 245/40R23. Fit 235/45R23 and the needle stops telling the truth.
| Needle says | You are doing | Out by |
|---|---|---|
| 30 mph | 30.6 mph | +0.6 |
| 50 mph | 51.0 mph | +1.0 |
| 70 mph | 71.4 mph | +1.4 |
| 90 mph | 91.8 mph | +1.8 |
The odometer drifts with it. Ten thousand miles on the dial are really 10,199 on the road, so it under-counts by 199. At a true 70 mph the first turns 766 times a minute, the second 751.