I read a book by some post-war German historian a few years ago, and one fact quite astonished me: it was either prior or right after operation Typhoon (don't remember by now) that the Germans lost 85% of all the trucks they had at the start of the campaign.In the case of Germany what motorization it had was critical for the fast movements during the battle of France, but later when you invade the USSR with more than 3.000.000 men, the soviets sabotage all the railways and on top of that you don't have the spare parts to replace what limited motorized equipment you had the supply train has to rely on horses.
And obviously, for the summer 1942 campaign w/e was left by then, was withdrawn from Army Groups North and Center to be transfered to AG South (which maybe was already split into 'A' and 'B' by then, my memory fails me these days).