When I say “diesel Ram truck,” you probably think of the Cummins I6, which Dodge pickups debuted in 1989. The industrial-grade 6BT has become nearly synonymous with Ram trucks, but it’s far from the first diesel the pickups and SUVs manufacturer tried.Throughout the 1980s — and even the 1970s — you could order Chrysler Corporation trucks with a range of diesels built by other companies. But there may be a reason Ram’s first diesels have been forgotten. The Cummins Engine In Dodge Ram Trucks Is A Living Legend Mecum AuctionsIn 1989, the engineers at Dodge managed to cram a diesel I6 that Cummins had engineered for heavy equipment use into the Ram truck. The result sent shock waves through the industry, achieving a whole list of firsts.The Cummins 6BT-powered Rams were the first pickup trucks to advertise a 400 lb-ft diesel. They were also the first mass-produced with direct injection and, in 1994, the famed Bosch “p-pump.” Later (1991.5) they would become the first to offer a factory-installed intercooler. Then (2013) Cummins-powered Dodges would be the first pickup trucks to break the 30,000-pound tow rating. Finally (2019), the 6BT in the Dodge Ram would break the 1,000 lb-ft of torque record.Ram trucks stand proud and mighty atop the mountain of diesel supremacy and sneer down at silly puny Power Strokes and Duramaxes. But it wasn’t always this way. In fact, it was once the absolute opposite. By 1989 Dodge Rams Were Playing Catchup Bring a TrailerThe Chevrolet C/K pickup trucks debuted a 5.7-liter Oldsmobile-engineered diesel V8 in 1978. GM really hit its stride with the 6.2-liter Detroit Diesel V8 in 1982. Meanwhile, Ford cozied up to International Harvester. It debuted the 6.9-liter indirect injection (IDI) V8 in its pickup trucks for the 1983 model year.Diesel fuel was relatively cheap and plentifully available. NOx emissions weren’t nearly as regulated as they are today, so these diesel engines could make a ton of power and achieve epic mpg. In the 1980s, motor heads in the know were falling in love with their diesel-powered GMs and Fords. But the Dodge Ram couldn’t get into the party without a date.Once International Harvester and Detroit Diesel had dance partners, there weren’t many diesel manufacturers left for Dodge to court. To make matters worse, it had already burned through two options. The Forgotten First Diesel Used In Full-Size Dodge Trucks: Ram 150/200’s Mitsubishi 6DR5 Bring a TrailerBy 1970, Chrysler Corporation had a problem. It was selling plenty of V8-powered muscle cars, but many markets were shifting to smaller, sportier, and more fuel-efficient vehicles. So, Chrysler bought a 15% stake in Mitsubishi, and began importing the Japanese automakers vehicles, rebadging them, and selling them through dealerships. (Think Dodge Colt and Plymouth Champ). Mitsubishi got to up its sales numbers while Chrysler could fill a gap in the market, building brand loyalty.Twin gas crises (in 1973 because of an embargo and again in 1979 because of an Iranian Revolution) caused fuel prices to rise and threatened pickup truck sales. Dodge’s solution was an efficient Mitsubishi engine in its full-size trucks. The lackluster performance of Dodge's first diesel truck Bring a TrailerThe 1978 Dodge truck borrowed a four-liter inline six-cylinder diesel that Mitsubishi had been using in big cab-over professional grade “Fuso” trucks in other markets. The engine might have been quick enough to keep up with traffic in Japan. But in the U.S. in 1978, it reportedly didn’t cut the mustard.The 105 horsepower/169 lb-ft engine made less power than the Dodge Slant Six, the entry-level choice in the same Ram. A Curbside Classics post claims 2,835 diesel Rams were built, but few forum members have ever met someone who owned one. Perhaps they all went to efficiency-minded fleet vehicle owners and were driven into the ground. Dodge dealerships unceremoniously deleted the engine from the 1979 options list. The Ram 50’s Mitsubishi 4D55 A Pint-Sized Diesel For A Pint-Sized Dodge Bring A TrailerIf at first you don’t succeed, try and try again. Right? Dodge and Mitsubishi took another swing at an efficient diesel truck with the Ram 50/Mitsubishi Mighty Max. I love these little Toyota-sized Mopars, and the idea of one with a turbocharged four-cylinder diesel is pretty nifty. But again, they didn’t quite do the trick.The result was an 86-horsepower compact truck. It couldn’t haul much and, depending on configuration, had a tow rating as low as 3,000 pounds. But the kicker is that its efficiency wasn’t that great either.The compact, diesel, FWD Volkswagen Rabbit pickup truck was getting 40+ mpg in 1985. Many owners reported 55-60 highway mpg. Meanwhile, the diesel Mitsubishi/Dodge got an EPA rating of 24 city/28 highway mpg (26 combined). Dodge pulled the plug on this engine after just three model years. Chrysler Killed A Diesel Jeep Truck JeepHow about a diesel-powered Jeep truck? No, it’s not a hot rod build done by a Cummins-loving Chrysler fan. Jeep actually offered a diesel engine in its Cherokee and the Comanche pickup truck which shared that SUV's running gear. I’ve encountered these 4WDs abroad, but in the U.S. they’re rare as hen’s teeth. For a reason.After the financial success of the world’s first minivan (the 1984 Dodge Caravan/Plymouth Voyager), Chrysler exec Lee Iacocca went on a buying spree. The “Chrysler comeback kid” who had once spearheaded the Mustang for Ford picked up some interesting nameplates. They included Lamborghini and Jeep (the latter via buying American Motors Corporation).In 1987, Jeep had a pretty cool lineup and has turned out to be a great investment. Since then, the CJ/Wrangler has all but printed money for Chrysler year in and year out. But Jeep presented Iacocca with a problem. It was building both the full-size J-10 pickup truck and the midsize Comanche truck. The Abrupt End Of The 1980s Jeep Pickup Trucks What’s more, the Comanche had just introduced a Renault diesel. The tiny 2.1-liter turbocharged engine maxed out at 132 lb-ft. While a gas-powered 4.0-liter Comanche could tow 5,000 pounds, its diesel brother was rated at 1,400-2,000.This wasn’t a workhorse engine. In fact, the Renault diesel was so weak you couldn’t option it with an automatic transmission. Despite all this, 2WD diesel Comanche fuel efficiency topped out at 24 city/29 highway mpg (26 combined). The 4WD couldn’t beat 26 highway mpg.The diesel Comanche was far from a towing beast. But with the Cummins/Ram project in the works, a diesel Jeep truck might have looked a lot like competition for Dodge. Chrysler killed all Jeep trucks within one model year of buying the company.