I own a 2016 GM Dually, so I'll throw my thoughts in on this. For one thing GM doesn't offer the 4.10 rear axle ratio for the Duramax equipped trucks. The only gear ratio available since the start of the Duramax trucks has been 3.73. The other thing I think is the Allison transmission gear ratios. Ram (Aisin) and Ford have much deeper transmission ratios than GM's Allison. Although the Allison has a much better reverse gear ratio. With the new towing "J2807" (don't know if that is the correct #) requirements, the higher geared GM has a big disadvantage getting started on the hills with the bigger loads that Ram and Ford have their trucks rated at. Just my thoughts on it.
