In the name of upkeeping their proud military tradition, your society has chosen one of the worst civil vehicles possible, rivalled only by steamrollers and drag cars.
* Tanks are incredibly uncomfortable to drive for extended periods of time, by the virtue of being engineered for being heavily armored and carrying guns. They lack significant ventilation, are often loud, and possess a multitude of hard edges for you to bang your head on. The king of this was the T34, due to its sloped armor.
* For the reasons listed above, they are also inept at carrying cargo. If you want to take a new TV or Lounge home, you either have to pray to god that it fits inside the commanders turret and comes back out in one piece, or lash it to the outside of the vehicle.
* Only fit healthy people can use it. Tanks are difficult to get out of, requiring you to clamber out of a turret to do so. An elderly person with movement difficulties, disabled person, or pregnant woman would have a heck of a time trying to get out.
* Tanks have terrible visibility. 347
Due to having few places to be shot at from, they also have few places to see from. Unless you are carpooling with someone in the commander's seat, you have terrible visibility. Considering you are driving a vehicle capable of ploughing through a house with minimal effort, urban commuting will result in a multitude of streetsign-flattenings, house-flattenings, pedestrian-flattenings, cyclist flattenings, heck, just about a flattening of everything else a few dozen tons of steel on treads can crawl over.
* The infrastructure required to allow widespread usage of 30-40 ton vehicles is nuts. Bridges, highways, parking lots -Tanks are inherently less reliable than cars and require much more effort to perform maintenance. Also, what will you do if a 30 ton vehicle breaks down somewhere inconvenient?
If you still want to keep a military-vehicle based society, I can offer a better alternative:
Half tracks, while capable of being militarised, due to their protective armor and varied weapon platform capability, are also a valid civil vehicle, having a far greater cargo capacity, visibility, and maneuverability. By virtue of being less of a heavyweight (8 tons to the Shermans 30) they are also inherently less expensive. They also have greater parts compatibility with actual cars, which you have stated also exist in this society. 87