Another 12 trucks have been discovered with tampered speed limiters during a two-state police probe into dodgy practices in the industry after a truck was clocked doing 142km/h on the Hume Highway.

Operation Overland was launched into Scott's Transport Industries on Monday.

Ninety-eight of the South Australian transport company's fleet of 322 heavy vehicles have since been intercepted for mechanical inspection.

Yesterday, police said they had found six trucks with tampered speed limiters.

Today, 12 more had been discovered, taking the total to 18.