315/45R18 vs 295/45R18
Going from one to the other changes the overall height by -2.4 %. That is inside the 3% band, so it fits and the speedometer stays close.
Side by side
| Measure | 315/45R18 | 295/45R18 | Change |
|---|---|---|---|
| Overall height | 29.2" | 28.5" | -2.4 % |
| Sidewall | 5.58" | 5.23" | -6.3 % |
| Section width | 315 mm | 295 mm | -6.3 % |
| Circumference | 91.6" | 89.4" | -2.4 % |
| Revs per mile | 692 | 709 | +2.4 % |
| Axle height | 14.58" | 14.23" | -9.0 mm |
| Approved rim width | 8.5–11.5" | 8.5–11" | overlap 8.5–11" |
Both are approved on a rim 8.5" to 11" wide. If yours is in that band, this is a tire change and nothing else.
What the speedometer will say
Calibrated for 315/45R18. Fit 295/45R18 and the needle stops telling the truth.
| Needle says | You are doing | Out by |
|---|---|---|
| 30 mph | 29.3 mph | -0.7 |
| 50 mph | 48.8 mph | -1.2 |
| 70 mph | 68.3 mph | -1.7 |
| 90 mph | 87.8 mph | -2.2 |
The odometer drifts with it. Ten thousand miles on the dial are really 9,757 on the road, so it over-counts by 243. At a true 70 mph the first turns 807 times a minute, the second 827.