The Germans, thank goodness, had several problems!!
1. They did not switch over to a full wartime economy until 1943-44!! They were still producing civilian items into '44.
2. There government was extremely fragmented, with hundreds of little fifedoms, each with its sacred manpower and/or manufacturing area, that no one else could touch, creating a huge waste of manpower, time and resources.
3. The Germans never standardized their trucks - numerous companies continued to build their own models of trucks, creating a huge spare-parts problem. Also many of their trucks were militarized civilian models - and tens-of -thousands littered Russia as they out ran their supply chain. The trucks wore out faster than they could build replacements. The USA standardized, for example, the 2-1/2 ton truck. There were minor differences in the cabs, and maybe small differences in engines, but basically there were the same truck built by several different companies (the same for the Sherman tank). The US and British militaries also built special trucks for military use, minimizing front-line use of "commercial" models.
4. Many Germans were farmers and used horses on their farms, Americans were in middle of a mechanization of the economy, and most young men knew how to operate and maintain vehicles - even farmers. The Americans were better at maintaining their equipment than their German counterparts.
5. The German bane, even today - they over-engineer everything!!!! Mercedes, BMW, Opel, etc., made great road vehicles, but it took specialists to maintain and repair them. See #4 above.