295/60R18 vs 315/60R18
Going from one to the other changes the overall height by +3.0 %. That is inside the 3% band, so it fits and the speedometer stays close.
Side by side
| Measure | 295/60R18 | 315/60R18 | Change |
|---|---|---|---|
| Overall height | 31.9" | 32.9" | +3.0 % |
| Sidewall | 6.97" | 7.44" | +6.8 % |
| Section width | 295 mm | 315 mm | +6.8 % |
| Circumference | 100.3" | 103.3" | +3.0 % |
| Revs per mile | 631 | 613 | -3.0 % |
| Axle height | 15.97" | 16.44" | +12.0 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 295/60R18. Fit 315/60R18 and the needle stops telling the truth.
| Needle says | You are doing | Out by |
|---|---|---|
| 30 mph | 30.9 mph | +0.9 |
| 50 mph | 51.5 mph | +1.5 |
| 70 mph | 72.1 mph | +2.1 |
| 90 mph | 92.7 mph | +2.7 |
The odometer drifts with it. Ten thousand miles on the dial are really 10,296 on the road, so it under-counts by 296. At a true 70 mph the first turns 737 times a minute, the second 716.