One thing I'd like to see you add is a pure-white version of each vehicle, so I can use my game engine's color tinting feature more easily. As it is, I have to load them into Photoshop and play around with various overlay layers to get something close to white, then usually redraw some of the details that faded out in the process. I know there are other ways to do this, but this is the method I know.
I made some alterations to my pack (the 4x version) to make white versions of all the semi-tractors, as well as a yard-hostler tractor and a couple of properly scaled 53' semi-trailers.
Awesome set of vehicles. Planning to use them in a semi-truck parking game I have in mind but haven't gotten around to doing yet.