285/45R18 vs 295/40R18
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 | 285/45R18 | 295/40R18 | Change |
|---|---|---|---|
| Overall height | 28.1" | 27.3" | -2.9 % |
| Sidewall | 5.05" | 4.65" | -8.0 % |
| Section width | 285 mm | 295 mm | +3.5 % |
| Circumference | 88.3" | 85.7" | -2.9 % |
| Revs per mile | 718 | 739 | +2.9 % |
| Axle height | 14.05" | 13.65" | -10.3 mm |
| Approved rim width | 8–10.5" | 8.5–11" | 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 285/45R18. Fit 295/40R18 and the needle stops telling the truth.
| Needle says | You are doing | Out by |
|---|---|---|
| 30 mph | 29.1 mph | -0.9 |
| 50 mph | 48.6 mph | -1.4 |
| 70 mph | 68.0 mph | -2.0 |
| 90 mph | 87.4 mph | -2.6 |
The odometer drifts with it. Ten thousand miles on the dial are really 9,713 on the road, so it over-counts by 287. At a true 70 mph the first turns 837 times a minute, the second 862.