GM Duramax Allison 5 to 6 Speed Conversion: The Ultimate Highway Upgrade

If you own an 01-05 Duramax with the 5-speed Allison transmission, this one is for you. The LB7 and LLY Duramax engines are held in high regard because of their reliability and lack of (or early) emissions equipment. These trucks are known to go hundreds of thousands of miles completely stock and maintain great reliability when upgraded and tuned. For many diesel enthusiasts—myself included—the early 01-05 Duramax was our first diesel truck.

The 5-Speed Limitation on Arizona Freeways

These trucks are great around town, and the Allison 5-speed transmission holds a decent amount of power while towing reliably. The only issue arises when you hit the freeway. At Power Forge Diesel in Yuma, Arizona, we know the drive to Phoenix or Tucson well, where speed limit sits at 75 mph.

The stock 5-speed Allisons aren't comfortable much above 75. At that speed, you’re looking at roughly 2200 RPM. Even if your temps look fine, nobody likes hearing the engine scream at those high RPMs for hours. It increases turbo pressure, raises EGTs (Exhaust Gas Temperatures), and puts the truck in a high-load situation for hours that most drivers would rather avoid.

Why the LBZ 6-Speed is the Gold Standard

A big reason the 06-07 LBZ Duramax is so popular is its 6-speed Allison transmission. However, swapping an entire 5-speed for a 6-speed Allison isn't in the budget for most of us. Many owners simply accept that they can’t drive highway speeds while keeping engine load low.

But here is what most people don't know: The Allison 5-speed actually has the gear ratios for a 6th gear built into it already. Whether it was due to tech limitations at the time or manufacturing costs, Allison originally equipped these transmissions with a 5-speed valve body instead of a 6-speed valve body.

The Calibrated Power 6-Speed Conversion Kit

Calibrated Power solved this issue entirely. Instead of a full transmission swap, they produced a 6-speed valve body conversion kit and a new TCM (Transmission Control Module) with custom tuning.

• No Transmission Swap Required: You keep your existing Allison.

• RPM Reduction: This kit reduces engine RPM by 14%.

• Real-World Stats: It’s the difference between 75 mph at 2200 RPM versus 75 mph at 1800 RPM.

• Customized Tuning: The TCM is calibrated to your specific truck, accounting for tire size, lifts, and engine upgrades.

Proven Reliability in Yuma, AZ

You might wonder about the long-term reliability. I (Spencer, founder of Power Forge Diesel), have run this exact kit on my 01 LB7 dually for years. I’ve used it for heavy towing across the country many times and, honestly, I’ve abused it. It has held up perfectly.

At Power Forge Diesel, we’d rather help you install a quality upgrade that lowers your RPMs, decreases trans & engine temps, lowers EGT's, and improves fuel economy than see you back in the shop for a heat-related engine failure or a full trans rebuild. This is one of those rare upgrades that eventually pays for itself through better fuel economy and reduced wear and tear on your powertrain. If you’re looking to modernize your 01-05 Duramax for the Arizona highways, we highly recommend the 6-speed conversion.

Link to 5-6 gear conversion:

duramaxtuner.com

2001-2005 LB7/LLY Duramax 6 Speed Conversion Kit

Want to talk gears? Call Power Forge Diesel in Yuma at (928) 888-9685 to see how a 6th gear can change how your LB7 or LLY drives.