:exactly: The Allison should have been a 6 speed when they first came out with the Duramax. I'll drop mine down into 5th every now & then on the highway, & it just seems like the engine is running way too fast. I can't imagine what it would be like driving my old LB7 with the 5 speed Allison now that I have an LBZ. (And I never had any complaints with the LB7/Allison combo when I still owned it.)6th is beautiful on the highway. 70mph and the engine is purring at @ 1750rpms. Very easy to speed with.
650 lb/ft of torque at 1600 rpm is pretty much all you are going to get. maybe your tach is wrong . pulled with pretty much all models 6.9 ford, 7.3 non turbo, 7.3 turbo, 6.0 ford and even dodge cummins 2005, and the duramax is by far the strongest light truck i have ever towed with.( not sure if light truck is right terminology but i am sure someone will correct me if i am wrong )The only thing that I would change would have to be the closeness of the lower gears. I try to baby it until 5th and the gears are just too close, 200-300 RPM difference. Id prefer there to be 600ish, But once I hit 5 bam good amounts of power. Mind you there is plenty there in the low gears, but i want the 1250-1400 RPM building power vs 1700-1800 RPM power.