Six men from the West Midlands and one from West Yorkshire were detained over three days, West Midlands Police said.
They said the car was stopped on the M1 expressway in South Yorkshire on Saturday - a search later found firearms, offensive weapons and other material inside.
The men were being detained in the West Midlands area "on suspicion of the commission, preparation or instigation of acts of terrorism", police said.
The car had been impounded on suspicion of having no insurance.
Police traced and arrested the driver, passenger and others on suspicion of the commission, preparation, or instigation of acts of terrorism.
The BBC reported that the arrests are not linked to the 2012 Olympics or the arrest of six terror suspects in London yesterday.
Items found in the vehicle were undergoing forensic analysis, and searches were carried at the addresses of those in custody.
"As soon as the items were discovered in the impounded vehicle, our priority was to protect the public by pursuing and arresting those we believed to be involved," head of the West Midlands counter-terrorism unit, Detective Chief Superintendent Kenny Bell, said.
