Let’s face it—Mopar hasn’t exactly built a reputation for bulletproof transmissions, especially when it comes to the heavy-hitting diesel pickup world. Ask any 6.7-liter Cummins owner and you’ll hear the same arguments over and over again: Do I go with the 68RFE or the Aisin? How do I even know which one I have? Did I screw myself by buying the “wrong” one? Can I swap between them? How much power can each handle stock? 

The truth is there’s no clear-cut winner and loser when it comes to the 68RFE versus Aisin debate. Each transmission has its strengths, weaknesses, and quirks. The real key is understanding where each one fits and what to expect.

We pulled up a chair and grabbed some coffee with Nate from NextGen Drivetrain and absorbed all the knowledge as he dove into the weird world of 2007 to 2024 Ram heavy-duty pickup transmissions.

Meet The 68RFE

This transmission really needs no introduction. The six-speed 68RFE showed up in 2007.5 as Chrysler’s answer to Ford and GM, who were already rolling out five- and six-speed automatics. The 68RFE gets a bad rap for being fragile in stock form. The torque converter can’t hang with Cummins torque, the valve body wears out fast, and when you throw power or big tires at it, it simply folds. Push past 400 horsepower without mods and you’re looking at a transmission obituary in just a few thousand miles. Beyond that? Instant failure.

But here’s the twist: when built properly, the 68RFE can be a solid performer. From tuning options to billet valve bodies like the Project Carbon series, the aftermarket world has the 68RFE covered. That’s why you’ll see builders putting together “SuperStock” 68RFEs for $6,000 to $7,000 that live happy, reliable lives behind tuned trucks.

The flip side? Every serious part inside has to be re-engineered for real performance, which is why building one right costs more than just about any other diesel pickup transmission. And let’s not forget—its roots go back to the Plymouth Caravan’s 45RFE. That’s right, the DNA of your heavy-duty diesel transmission traces back to a minivan.

Enter The Aisin

Now let’s talk about the Aisin Seiki, Mopar’s answer to the Allison. First appearing in 2007.5 cab-and-chassis Cummins trucks, the Aisin came in two flavors: the AS68RC (2007.5–2012) and the AS69RC (2013–present). You’ll know you have an Aisin if the transmission dipstick is on the driver’s side versus the passenger’s side for the 68RFE.

On paper, the Aisin sounds like a win—it was designed to compete with Allison, after all. But reality hit hard. The AS68RC was plagued with pump and valve body failures so bad that most cores were unsalvageable. Builders had to engineer fixes like the “Formula One” pump and valve body just to keep them alive. The AS69RC improved on that, but it still shares many of the same weak points.

The most notorious? The K2 hub. This critical component has a nasty habit of eating itself alive, even under stock towing conditions. No power adders required. Just hook up a gooseneck or run a welding rig and boom—you’re pulling the trans for a rebuild.

And then there’s the fuel de-rate. Every time the Aisin shifts, the engine pulls back power to spare the clutches. It works—sort of. That’s why you’ll hear stories about Aisins lasting 300,000 miles—but only when the truck was babied and never asked to do more than grocery runs and light towing. Take one to the track, and you’ll quickly realize it was never built for speed.

Worse yet, you can’t tune them. The electronics are locked down tight, which means no aftermarket TCM tuning, no faster shifts, and no real performance flexibility. That’s a huge minus compared to the 68RFE’s open playground of mods.

The Swap Question

So, can you swap from a 68RFE to an Aisin, or the other way around? Short answer: no. Long answer: you don’t want to.

These transmissions are completely different animals with separate hardware, control systems, and electronic languages. It’s like trying to play an Xbox game on a PlayStation—doesn’t work, won’t work, and never will. Even upgrading from the AS68RC to the AS69RC in earlier trucks isn’t feasible because of the electronics gap.

68RFE Versus Aisin: The Bottom Line

Here’s the straight talk: if your truck came with a 68RFE, build a better 68RFE. If it came with an Aisin, build a better Aisin. Swapping isn’t realistic, and every platform has proven aftermarket solutions—yes, even the “unicorn” AS68RC.

If you’re shopping, steer clear of the AS68RC. It’s just too problematic. But between the 68RFE and the AS69RC, either can be built into a strong transmission if you’re willing to invest. Both will need serious upgrades to handle big power or long-term abuse, so budget accordingly.

At the end of the day, no matter which transmission your Cummins came with, it can be made reliable. And when you’re ready to build it right, that’s when companies like NextGen Drivetrain come in with solutions tailored to each platform.