Onto which trucks will be ingame. Unfortunately, there's something called licensing, and for now, only 2 trucks will be available at launch. Honestly, I kind of was expecting this, as when Euro Truck 2 first launched it also only had 2 licensed trucks, and it took years untill they finally had Mercedes onboard.
Anyway, the two starting trucks will be:
The Kenworth T680
The Peterbilt 579
As why SCS only managed to get licences for two trucks so far, SCS stated the following:
Licensing talks are often resembling the "which came first, the chicken or the egg?" problem. To obtain a license in a highly specialized field, you just don't walk into a store and get it over the counter. Unlike car manufacturers who have extensive experience with being featured in racing games, for most truck manufacturers this is all unexplored territory. We always need to find a way inside the company to reach the decision makers at marketing/licensing department, and getting our voice noticed is not easy. Just getting the chance to get out point across is often all it takes, and with American Truck Simulator out in playable form and in the hands of customers, it gives us a crucial talking point.
Fortunately, SCS is in the finalizing stage of licensing talks for the following trucks, which will most likely be released soon after game release:
The Kenworth W900
The Peterbilt 389
As with ETS2, any new trucks will be free updates!
Next to that, SCS has tried getting a foot in the door at Freightliner, International Lonestar and Volvo (VNL), but SCS explicitly states there is no guaruantee whatsoever that they can manage to get a license for these trucks, so there is no guarantee they will make it in the game. However, SCS has made models already in more or less finished states, but for now they only serve as an instrument to be used for getting a better chance on a license. For the full explaination read more at the
SCS blog.