Copied what I wrote in another thread:
Generally I find US late game to be strong only if you keep your armour alive. If you keep making shermans and losing them piece meal you will lose badly. At all cost keep all armour alive and when you have 3-4 tanks you can use one smoke barrage and then flank and charge with all your tanks and vetted infantry. That's how I've been winning late game against OKW. You shouldn't be worried about sitting back with your armour as long as you keep a decent fuel income as OKW doesn't get as much fuel as you. If the OKW gets jagdpanzer 4 or higher armour tanks then you might want to switch up a sherman with a m36 jackson.
US tanks do a lot of damage but can't take a lot of punishment in return. As long as you keep your armour alive things will start to snowball and you can defeat any armour the Germans throw at you. Thanks to the resource scarcity of OKW you should be able to field far more tanks than the germans so long as you keep your armour alive.