315/30R18 vs 295/35R18
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 | 315/30R18 | 295/35R18 | Change |
|---|---|---|---|
| Overall height | 25.4" | 26.1" | +2.7 % |
| Sidewall | 3.72" | 4.06" | +9.3 % |
| Section width | 315 mm | 295 mm | -6.3 % |
| Circumference | 79.9" | 82.1" | +2.7 % |
| Revs per mile | 793 | 772 | -2.7 % |
| Axle height | 12.72" | 13.06" | +8.8 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 315/30R18. Fit 295/35R18 and the needle stops telling the truth.
| Needle says | You are doing | Out by |
|---|---|---|
| 30 mph | 30.8 mph | +0.8 |
| 50 mph | 51.4 mph | +1.4 |
| 70 mph | 71.9 mph | +1.9 |
| 90 mph | 92.4 mph | +2.4 |
The odometer drifts with it. Ten thousand miles on the dial are really 10,271 on the road, so it under-counts by 271. At a true 70 mph the first turns 925 times a minute, the second 900.