There's an interesting thing about early Chevrolet Silverado and GMC Sierra trucks equipped with the Duramax diesel engine and Allison transmission. From 2001-2005, these trucks were equipped with a five-speed Allison automatic transmission. In 2006, the trucks received a big update, a "new" six-speed Allison transmission. The interesting discovery came after the new transmission was torn down for the first time. It quickly became apparent that the internals were, for the most part, exactly the same.

That's right, the 2001-2005 Allison five-speed transmission was actually a six-speed all along. The difference was in the valve body and the transmission control module (TCM). Much like the legend of Big Foot, it's been told that the reason for not utilizing all six gears from the start was a lack of processing power in the TCM, taking until 2006 for the controller hardware technology to catch up to the hardware. Another story frequently told is that it took a disgruntled Allison employee to smuggle out the early TCM code and make it available to tuners.


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