2020 Ram 2500 Cummins Diesel Yearlong Test: We Get 895 MPG…

“Four tons of exceptionally capable and remarkably comfortable truck never seemed so thrifty.”

Our long-term 2020 Ram 2500 Cummins, Guffman, is gunning for MotorTrend‘s long-term vehicle mileage record. In its first 299 days with us, it accrued 23,288 miles. Most of these were transporting people to places they’d have flown to in more normal times. Every long-hauler has marveled at the impressive fuel economy and range numbers displayed on the driver information center of this 7,980-pound monster. We’ve seen highway fuel economy numbers as high as 23 mpg and range estimates of 645 miles or more. How do these numbers square with reality?

On a 1,600-mile run from Detroit to Memphis in April, I decided to probe as much of that estimated range as possible. The “low fuel” lamp illuminated with an estimated range of about 70 miles remaining (500 miles after fill-up), triggering the native nav system or CarPlay apps to ask if you’d like to search for a station. The indicated remaining range number gets replaced with the word “low” at about 15 miles remaining, by which time the fuel gauge needle had dropped below “E.” (Thankfully, it’s an analog needle, which I find more reassuring than an electronic gauge.)

In my formative years, I owned a 1980 VW diesel and was frequently cautioned never to run it out of fuel, because priming a dry high-pressure fuel pump was a serious PITA. Without having researched the priming procedure for our Cummins, I chickened out at 566.8 miles. But then I only managed to squeeze 28.144 gallons of diesel into the 32.0-gallon tank (at which point a few ounces overflowed to suggest it was indeed full). The math works out to 20.1 mpg—about 3 mpg shy of the computer reading, and apparently 77.5 miles shy of needing to consult the pump priming instructions on page 428 of the owner’s manual.

Turns out Ram probably doesn’t need to be so conservative with the gauge, because Ram puts an electric “lift pump” in the tank that effectively pushes the air out of the lines leading to the high-pressure pump in the engine compartment. Here’s the priming procedure: Put 2 to 5 gallons of fuel in the tank and then switch the ignition to run (not start) and then back off again—for 30 seconds each—three times. Then start the engine and allow it to idle for 30 seconds before driving.

While we’re talking fuel, the Ram 6.7-liter Cummins is designed to burn blends of up to 20 percent biodiesel, and the mostly Midwestern fuel we’ve purchased—from top-tier brands wherever possible—has typically measured between B2 and B20. The internet is full of cautionary tales about cheap fuels or excess biodiesel causing problems.

Of course, Guffman doesn’t drink diesel neat; he requires a misting of diesel exhaust fluid, a.k.a. AdBlue or DEF. In our largely unladen highway-cruising-biased duty cycle, a gallon of DEF lasts 895 miles. It’s easy to monitor this tank via the matching analog DEF gauge that sits opposite the fuel gauge. (Ram’s heavy-duty diesel competitors rely on electronic gauges you must activate.) Running out of DEF can prevent the engine from restarting. We’ve learned to add DEF during highway trips, as truck stops stock the handier, more economical 2.5-gallon DEF refill containers, while city stations often only carry the 1-gallon bottles, sometimes without a filler hose.

One Ram DEFiciency: The small, round fuel filler door makes it hard to add DEF while you’re dispensing 20-plus gallons of diesel. Just budget time for a hand-washing stop—the DEF refill is frequently messy and smelly, because the hose attachment that comes with most DEF containers generally leaks a bit of this ammonia-based stuff.

In all, we’ve spent $0.16/mile on diesel and $0.01/mile on DEF—numbers that would clearly rise had we been hauling closer to the 2,020-pound payload limit or towing nearer the 19,020-pound maximum for a significant number of those miles. Then again, if that was our duty cycle, we’d have ordered a 3500 model. We’ve worked Guffman hard, and he’s performed admirably.

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