Full is 5 gallons.
That makes each 1/4 = 1.25 gallons.
Don't overfill regardless of ambient temps, the sensor is sonic and can register false/"stick" if overfilled. Though I think there was a TSB that fixed that stuck reading issue.
There's two schools of thought on DEF tank level during below 12F weather when it can begin to freeze.
- Keep it topped off to reduce the likelihood of freezing (more mass=less chance)
- Keep it low to prevent a solid frozen 5 gallon chunk of DEF that will never unthaw unless parked in a heated garage 24-48 hours
I'm in the #2 camp.
First, the software will not put the truck in limp mode if the DEF is frozen and can't initially pump (to a point).
Second, I can add fresh DEF to assist with the unfreeze.
This is all, of course, going to depend on weather forecast and use/non-use patterns that are unique to every owner. I'm not going to add more DEF if the truck's just going to sit parked below freezing.