Just read about this transmission - turns out it isn't a 4 speed auto! At least, in typical operation it isn't. It has three orbital gearsets, similar to how a Prius works, and no torque converter.
https://www.caranddriver.com/reviews...rid-road-test/
Quote:
This Tahoe is the first “full hybrid” from General Motors, meaning that it can run purely on electric power at low speeds. It uses a new two-mode hybrid system that is a joint development of GM, BMW, Chrysler, and Daimler and is fundamentally different from the hybrid powertrains used by Toyota and Honda.
The transmission resembles a conventional four-speed automatic to which two 80-hp electric motors are integrated by marrying them to the transmission’s three planetary gearsets. Despite the two-mode name, the transmission, which fits into the same space as the automatic in conventional Tahoes, seems to operate in three distinct modes.
The first mode is continuously variable (CV) and provides ratios from infinity to 1.70. The Tahoe always starts in this mode because it lets the transmission take off smoothly from a stop without the need for a torque converter. When accelerating gently, you can stay in this mode using only the electric motor up to 32 mph, but usually, the 6.0-liter V-8 cuts in to provide additional power.
The second mode also uses CV operation but with a different planetary gearset to provide ratios from 1.70 to a deep overdrive of 0.50 (used during coasting and regenerative braking). This mode provides the benefits of a CVT at higher speeds while keeping the electric motors running in their most efficient rpm range.
Finally, the transmission has four fixed ratios, which are selected when the electric motors are needed to charge the batteries or when towing heavy loads. The powertrain management system, which GM calls the “hybrid optimizing system,” juggles the mode and the ratio to provide the most efficient operation at each driving load.
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The fixed ratios are as follows:
CVT1 - 0-1.70
CVT2 - 1.70-0.50
1st - 3.69
2nd - 1.7
3rd - 1.0
4th - 0.73
Rear ratio : 3.08
In 4th gear it's turning 1800rpm @ 75mph - which is still pretty great! However this is not how it typically operates. When running continuously variable ratios (which is whenever the batteries aren't completely flat) it can drop down to as low as 1230rpm at 75mph.