305/35R20 vs 315/35R20
Going from one to the other changes the overall height by +1.0 %. That is inside the 3% band, so it fits and the speedometer stays close.
Side by side
| Measure | 305/35R20 | 315/35R20 | Change |
|---|---|---|---|
| Overall height | 28.4" | 28.7" | +1.0 % |
| Sidewall | 4.20" | 4.34" | +3.3 % |
| Section width | 305 mm | 315 mm | +3.3 % |
| Circumference | 89.2" | 90.1" | +1.0 % |
| Revs per mile | 710 | 703 | -1.0 % |
| Axle height | 14.20" | 14.34" | +3.5 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 305/35R20. Fit 315/35R20 and the needle stops telling the truth.
| Needle says | You are doing | Out by |
|---|---|---|
| 30 mph | 30.3 mph | +0.3 |
| 50 mph | 50.5 mph | +0.5 |
| 70 mph | 70.7 mph | +0.7 |
| 90 mph | 90.9 mph | +0.9 |
The odometer drifts with it. Ten thousand miles on the dial are really 10,097 on the road, so it under-counts by 97. At a true 70 mph the first turns 828 times a minute, the second 820.
What drivers reported
Nobody has filed a complaint naming 305/35R20. 315/35R20 carries 2.