Criticism of American tanks was already prevalent during WWII. Despite details about tanks being heavily classified, many people, especially soldiers, were aware of the fact that German tanks were bigger and heavier than the Sherman, mounting higher caliber guns. Of course, the Tiger, which many thought was the default German tank, was a very rare encounter, but that was also a fact hidden by wartime secrecy.

For example, the San Antonio Light newspaper ran an article titled "Citing claims of German superiority, probe of US weapons demanded" in 1945, claiming that "the improved German 88-mm gun is equal or better than any American gun", a claim that would not be out of place on the internet today. The mistaken opinion that the heavier a tank is the better. The article found its way to the Tank Destroyer Board, where the claims were torn to shreds by people who actually knew what they were talking about.

Major Elston of the Tank Destroyer Board rebuffs the claim with the following points:

• Heavier is not always better. As he put it, "The best tank is the winning tank." He gives an example of the Ferdinand, the heavier German vehicle, being very slow and already out of production.

• In actual encounters with German tanks, American tank destroyers typically achieve a 5:1 advantage in kills, rather than the reverse, which is commonly claimed today.

• Having to land on beaches, medium tanks are a much more useful investment than heavy tanks.

• While the "German 88 mm gun" had more muzzle energy, it was an atypical weapon, and the American 76 mm gun compared favourably to German 75 mm guns.

However, the general opinion that the enemy's tanks were somehow better prevailed. In 1974, you still see magazines like ARMOR complaining about the inadequacy of the Sherman's 75 mm gun. Allegedly, the gun was already inadequate in Africa, back when the most armour you would see on a German tank was 50 mm, and the Tiger was not even in service.

Then we get to the biggest driver of the Sherman myth. The book Death Traps, published in 1998, is still very relevant in popular culture, and was (and is) sold with great success, despite its questionable value as a historical source. Not only is using memoirs a shaky proposition at best, but Cooper goes on to describe events that he was not, and could not be, present for, as well as getting elementary details about mechanical aspects of the Sherman wrong.

Sources:

https://worldoftanks.com/en/news/chieftain/The_Cheiftains_Hatch_Sherman_PR_Bigger_Cooper/

https://tankandafvnews.com/2017/09/02/from-the-vault-general-patton-and-the-sherman-gun-debate/

https://tankandafvnews.com/2015/01/29/debunking-deathtraps-part-1/

Can you provide some context by listing some examples you've seen?