I just opened and messed with this guy over the weekend, and I cannot for the life of me get his rear wheels to sit straight...the rear of the combined truck is at a slightly elevated angle facing upwards at the rear.

Is there a trick I'm missing here, or is it just how the mold is?

Either way, it's my first experience of the Siege Magnus-&-friends mold and I quite like it for Nova Prime. The truck mode in particular works a lot better for the character than I was expecting.