What the F is DEF?
DEF stands for Diesel Exhaust Fluid. It reduces the amount of nitrogen oxide in the exhaust of a diesel powered engine. It began being implemented in US passenger vehicles in 2010 and as far as we know, all trucks made today require DEF fluid. When purchasing a used vehicle it is worth considering the pros and cons of DEF equipped a vehicle.
How Much DEF?: On average DEF consumption is about 2% of diesel consumption. In other words, for every 25 gallons of diesel you will need 1 gallon of DEF. If you have a 25 gallon diesel tank and a 5 gallon DEF tank you'll need to fill your DEF every 5 fill-ups. If you run out of DEF your vehicle will likely going into "limp" mode after a few hundred miles. Limp mode means that your vehicle won't go over 5 mph.
Cost: At the time of this writing DEF costs about $2.80 per gallon at the pump and about $8 per gallon by the jug (typically sold in 2.5 gallon jugs). The following truck stops usually sell DEF at the pump: TravelCenters of America, Flying J, Love's, and Pilot.
Storage Space: If storage space in your truck or trailer is a concern, having to store one or more jugs of DEF might be a concern. If you know you'll always be close to a truck stop or retailer that sells DEF then you needn't worry but if you want to be prepared, you should probably always at least one jug of DEF on hand.
Fuel Economy: We don't really have a way to calculate this because we can't turn our DEF system on and off but studies show that DEF can improve mileage by 3-5%.
I don't know that if we knew more about DEF it would have influenced our truck purchasing decision. However, if you find yourself with two very comparable vehicles except that one requires DEF and the other does not, hopefully this information will help you make a better decision. What we wish we did know was the buying DEF by the jug costs almost 3x what it costs at the pump!