295/40R23 vs 285/45R23
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 | 295/40R23 | 285/45R23 | Change |
|---|---|---|---|
| Overall height | 32.3" | 33.1" | +2.5 % |
| Sidewall | 4.65" | 5.05" | +8.7 % |
| Section width | 295 mm | 285 mm | -3.4 % |
| Circumference | 101.4" | 104.0" | +2.5 % |
| Revs per mile | 625 | 609 | -2.5 % |
| Axle height | 16.15" | 16.55" | +10.3 mm |
| Approved rim width | 8.5–11" | 8–10.5" | overlap 8.5–10.5" |
Both are approved on a rim 8.5" to 10.5" wide. If yours is in that band, this is a tire change and nothing else.
What the speedometer will say
Calibrated for 295/40R23. Fit 285/45R23 and the needle stops telling the truth.
| Needle says | You are doing | Out by |
|---|---|---|
| 30 mph | 30.7 mph | +0.7 |
| 50 mph | 51.2 mph | +1.2 |
| 70 mph | 71.7 mph | +1.7 |
| 90 mph | 92.2 mph | +2.2 |
The odometer drifts with it. Ten thousand miles on the dial are really 10,250 on the road, so it under-counts by 250. At a true 70 mph the first turns 729 times a minute, the second 711.