The type of army tank that US Army Command Sergeant Major Zack Carey (James Garner') owned was a World War II era Sherman Tank, aka an M4 Sherman Tank. Previously known as the M4 Medium Tank, the tank is a medium tank, and was named after Union General William Tecumseh Sherman. Around 50,000 of them were manufactured, most of them around World War II. The tank was designed in 1940 and first produced in 1941.
"Tank" was originally rated R by the Motion Picture Association of America. However, a successful appeal was made which re-issued the motion picture with a PG rating instead. The film's 1983 release predated the PG-13 rating, for which this picture would've likely been rated had it existed when "Tank" was first released.
The Posse Comitatus Act mentioned in the movie is a real life American statute first assented in 1878. The act was passed to stop local, state and civil authorities from utilizing the military for law enforcement. It doesn't stop this happening, the authority to do so must come from the United States Constitution or Act of Congress and usually comes from the direction of the President of the United States.
The late Sherman tanks had a smooth front (aka glacis plate) but the tank in the film has bulges in front of the driver and assistant driver/hull gunner, so the tank seen in the film as an early model Sherman tank. The central fuel fill port and unique engine hatches in the back identify it as an A3 variant with the Ford GAA-engine gasoline engine, so thee tank is properly identified as an early M4A3 75mm gun tank with VVSS suspension.
When Sheriff Buelton demands $10,000, Sgt. Maj. Carey replies, "Sheriff, I don't know what you think the army pays." Effective 1 Jan 1984 a Sergeant Major with over 26 years of cumulative service time was paid a base rate of $2,303.70 per month. This does not include any housing allowance, bonuses, etc. that s/he might be earning.
The actor who played the Governor of Georgia, Wallace Wilkinson (1927-2001), shared his name, but no relation, with a man who would be Governor of Kentucky three years after the film's release, Wallace G. Wilkinson (1941-2002), who served a single term as Kentucky's Governor from 1987 to 1991, and had no record as an actor.