295/35R18 vs 315/30R18
Going from one to the other changes the overall height by -2.6 %. That is inside the 3% band, so it fits and the speedometer stays close.
Side by side
| Measure | 295/35R18 | 315/30R18 | Change |
|---|---|---|---|
| Overall height | 26.1" | 25.4" | -2.6 % |
| Sidewall | 4.06" | 3.72" | -8.5 % |
| Section width | 295 mm | 315 mm | +6.8 % |
| Circumference | 82.1" | 79.9" | -2.6 % |
| Revs per mile | 772 | 793 | +2.6 % |
| Axle height | 13.06" | 12.72" | -8.8 mm |
| Approved rim width | 8.5–11" | 8.5–11.5" | 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 295/35R18. Fit 315/30R18 and the needle stops telling the truth.
| Needle says | You are doing | Out by |
|---|---|---|
| 30 mph | 29.2 mph | -0.8 |
| 50 mph | 48.7 mph | -1.3 |
| 70 mph | 68.2 mph | -1.8 |
| 90 mph | 87.6 mph | -2.4 |
The odometer drifts with it. Ten thousand miles on the dial are really 9,736 on the road, so it over-counts by 264. At a true 70 mph the first turns 900 times a minute, the second 925.