Long winded indication of how much def to expect. DEF usage will be somewhere around 1.5 - 2% of fuel usage. 100 gallons of fuel used will take about 1.5 - 2 gallons of def. If you want to accurately determine DEF usage, the method needs to be the same as hand-calculating fuel consumption. Fill the tank. Record the odometer reading.
I think I pay around $2/gal bulk at the pump. I think the usage rate is 3 - 4% of the fuel burn rate. Pretty insignificant in my mind. My trip through the mountains of Idaho and Utah (1000 miles) caused me to burn about (round numbers) 100 gals of diesel and 3.5 gals of DEF.
For metal fuel tanks, the minimum that manufacturers aim for is 10 years of use. Now there will be some variance in how long each material might last individually, either coated steel or aluminum, but unless there is some external damage done to the tank then it will last at least a decade. Some tanks last for decades, so metal tanks only need
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