The diesel exhaust fluid (DEF) capacity of Your 2018 Dodge Ram 3500 iS5.5 gallons, which should last for about 7000 miles. You can refill this with the common AdBlue solution You find at truck service stations. You might think that You can just pee in this, since both urine and DEF contain urea, but the latter uses a far more concentrated amount: 32.5%. And yes, Your Dodge's sensors can actually detect it if You try. When You see the warning light that You won't be able to restart Your vehicle if You let this fluid get too low, take heed: It's not joking, and You won't be able to just put a little in, if You let it go dry: You'll need to completely fill the reservoir to be able to start Your truck back up.
Just like fuel mileage will vary, DEF use depends on your load and driving habits. I am towing a triple axle 19,000 lb. 5th wheel RV, and if I stay at 65 mph on trips I get 9 mpg and use approximately 1 gallon of DEF every 500 miles. This is a combination of flat and mountain driving. If I try to go 75 mph my mpg drops to 7 mpg and DEF usage goes up accordingly. No way will a tank of DEF last 7,000 miles unless you are running empty, and that's not what these trucks are built for, that's for a small 1500...
Hubert M
July 16, 2020
The 2018 ram 3500 def tank is able to hold 8 gallons of fuel.
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