On the initial fighting:

With the US facing most of the German armor instead of Britain I would have expected the US Army to much more quickly start moving towards upgunning their AFV with first the 90mm and second, the 17pdr. Additionally, the M4A3E2 Jumbo would have proven more valuable than it originally did. It would certainly have been a bit interesting to see the US dumping the Sherman in favor of the M 26 and possibly other even heavier tanks like the T 29 - 34 series.
As the US has far more resources to produce new AFV and already had designs moving towards production I would think the M 26 and M 36 would have been pushed into service alot earlier than they were.
I would also expect that the US would not have tried a Hill 112 or Goodwood type of set piece head on attack style of warfare. They likely would still have carpet bombed the Germans more than once but it is very unlikely that any US Army general would have tried a set piece attack more than once and likely would have been replaced if his first try was costly. It is not the US style of war to commit to the sort of attack Monty prefered.