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quote:I don't use it save for either extreme uphill or downhill grades (gain a bit of engine braking). It just works the engine too much for regular towing and kills mileage without really helping anything.
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This has been my practice for 15 years with three different Duramax/Allison 2500 models, our current one a 2007 year model. The tow/haul is a great feature. I wouldn't do without it, especially on the mountain passes in Colorado, but I see no reason to leave it engaged on level highways. Just my opinion and my experience, of course.