315/55R18 vs 295/55R18
Going from one to the other changes the overall height by -2.7 %. That is inside the 3% band, so it fits and the speedometer stays close.
Side by side
| Measure | 315/55R18 | 295/55R18 | Change |
|---|---|---|---|
| Overall height | 31.6" | 30.8" | -2.7 % |
| Sidewall | 6.82" | 6.39" | -6.3 % |
| Section width | 315 mm | 295 mm | -6.3 % |
| Circumference | 99.4" | 96.7" | -2.7 % |
| Revs per mile | 637 | 655 | +2.7 % |
| Axle height | 15.82" | 15.39" | -11.0 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 315/55R18. Fit 295/55R18 and the needle stops telling the truth.
| Needle says | You are doing | Out by |
|---|---|---|
| 30 mph | 29.2 mph | -0.8 |
| 50 mph | 48.6 mph | -1.4 |
| 70 mph | 68.1 mph | -1.9 |
| 90 mph | 87.5 mph | -2.5 |
The odometer drifts with it. Ten thousand miles on the dial are really 9,726 on the road, so it over-counts by 274. At a true 70 mph the first turns 744 times a minute, the second 765.