My 2015 Ford F350 holds 5 gallons of DEF. When the display says it is under 1/2 full, I go another 150 miles (it lies about being 1/2 full) and then I dump in a 2 1/2 gallon box of DEF. I get 2,000 to 2,500 miles on a box using the method. I find it hard to believe
Richard Blackwell is only getting 1500+ miles on a full tank.
I have only hit the Ford 500 miles to "limp" mode twice and both were sensor errors that self corrected by adding a gallon or less of DEF.