2500HD's also have an overload spring. Makes it ride a little stiff in the rear.I run Airlift bags with the on board compressor. I removed the overload leaf spring essentially making it the same as a 2500HD and run 20 psi in the bags unloaded for a lot smoother ride unloaded. When I hitch up the fifth wheel I run 60 to 70 psi in the bags and it rides nice and level with a decent ride. If you look at the towing capacities on the Chevy web site there isn't much difference between a 3500 and a 2500HD. The dealer told me that the overload spring was the only difference between the two trucks.