Due to the increased sales of diesel passenger cars and SUVs, including those recently caught gaming the emissions system, a greater number of traditional, on-highway filling stations now have diesel’s green pumps. However, pumps that offer diesel exhaust fluid (DEF) continue to be few and far between.

自2010年以来,许多要求柴油DEF,a mix of two-thirds water and one-third urea, to run their higher-emissions controls. As a result, highway truck stops stock DEF at diesel pumps, while automotive parts shops and dealerships offer DEF jugs at a higher price. As with fuel, jobsites also may have bulk DEF for on- and off-road vehicles.

Finding DEF in the wild can be a hassle, especially as diesels are designed to slip into a “limp” mode when the DEF tank runs dry. For that reason, some diesel-truck users keep a jug around, just in case.

We felt the pinch on Interstate 70, driving a one-ton Ram 3500 pickup (see story, above) to Indianapolis: The diesel-fuel and DEF gauges were low. We took the Ram past several highway work zones before filling up in Warrenton, Mo. We bought about 20 gallons at a cost of $2.09 per gallon, or roughly $41.

We were interested to see whether DEF was offered at the same local highway pump. There was none, so we had to keep hunting. Enter Flying J, a large truck stop a few miles away. It had DEF in the pump but only on the heavy-duty trucking side.

We filled up the DEF tank with about 4.5 gallons at $2.79 per gallon, or $12.56. We later filled up again with AutoZone’s 1-gallon DEF jug, which cost $8.01.