285/70R17 vs 315/70R17 is the metric version of 33 vs 35. Same 17-inch rim, same 70-series sidewall math, 30 mm more width. Height goes from 32.7″ to 34.4″. The 315 is still not a true 35. Compare them live: 285/70R17 vs 315/70R17.
The two stamps
| Size | Diameter | Width | Sidewall |
|---|---|---|---|
| 285/70R17 | 32.7″ | 11.2″ | 7.9″ |
| 315/70R17 | 34.4″ | 12.4″ | 8.7″ |
Diameter grows +5.1%. Width grows +10.5%. At 60 indicated on a truck calibrated for the 285, the 315 is doing 63.0 mph actual. That is a speedometer problem you will feel and a well problem you might hear.
What shops call them
285/70R17 is the stamp people treat as a 33 (32.7″). 315/70R17 is the stamp people treat as a 35 (34.4″). A 35x12.50R17 is the true 35, and it is also a 12.5 inch section. The 33 vs 35 page is the slang table.
A 315 on a wheel that was happy with a 285 needs more rim width and more room in the well. Diameter can pass the 3% check while the sidewall kisses a liner.