Switching from building tanks to trucks is not quite that easy for an industry like say the Soviet Union. At this point, most of it's industry in Siberia is closing down, moving back west where it is devestated (Hence one of the cheif reasons Stalin put the conquered states as his puppets, to rape them to rebuild his own nation). And even if they don't move their industry in the east, where most of their tanks come from, their factories are not built the same way as for example US Factories, which proved very adept at switching from cars to tanks. The Soviets could either produce a lot of tanks, or a lot of trucks, and they needed both in large numbers to continue any war.

But let's factor in other things, first, the Soviets are going to HAVE to devote a lot of their industry to be tooled up to build an Air Force, the Air Superiority the Allies will enjoy could be very decisive in the opening battles, and I don't think the Soviets will simply sit back with their current levels of Aerial production. Also, they face a dilemma of advancing US forces from the Pacific coming dangerously close to the industry that was safe from Germany, not too mention US bombing of major industrial areas. I'm not sure if they had the ability to rebuild destroyed industry as fast as Germany did, but it is still a major annoyance anyway, especially considering the major lack of railroads to transport made goods to the west, the Allied Air Force can simply destroy these few routes and be content that soviet equipment will sit in railyards, very far away from Western Europe.

Also, the US Suppplies a lot more then trucks to the Soviet War effort, at one time I had numbers, but I have since lost them and can't find a reliable source, but it was significant. Also the US sent the Soviet Union a huge amount of grain, Soviet farmlands had been destroyed by the war with Germany, Stalin had to disband a huge portion of his precious army shortly after 1945 becuase the US stopped offering it for free and he couldn't keep his population from starving and his troops fed, during war he can't afford this and the US is obviously not providing it for free anymore, so Stalin is facing a mass starvation of his manpower pool.

Also, you seem to be missing the point, trucks are just as valuable as a tank to the Soviet operational doctrine (It's called Deep Operation something or other, Doctrine I think), it involves a massive armored spearhead attack to break enemy defense lines, and then a huge number of trucks rush infantry to the gap to exploit it and widen it, and even more trucks to keep the infantry and armor in supply, and even more to keep general supply open, now if they are running low on trucks, the Armor breaks through, but the infantry has to march to the gap, and now enemy reserves are being brought up to strengthen the line, the Soviet infantry runs into a prepared enemy and has to retreat with heavy losses, and their armor force is now cutoff behind enemy lines, and armor doesn't last long without fuel.

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- Your freindly neighborhood cynic