The TU were production based small bore cars. They consisted of cars of limited series-production which may be submitted to certain modifications aiming at making them more adapted to competition.In order to simplify things. The category was dropped by the end of 1973 and TU cars became GTU cars.

Until 1973, the bulk of the entry lists was made of BMW 2002s, Ford Escorts, Datsun 510s and Alfa Romeo GTVs.

Proprietary engine block must be used; may be machined so long as camshaft location was not altered.

Bore and stroke could be altered not to exceed calculated 2.5L class limit.

Cylinder head could have same number and location of valves, ports and spark plugs.

Gearbox or transaxle were free but must remain in standard location.

Rear axle should remain live or independent as appropriate.

The standard body tub should be retained along with standard wheelbase.

All suspension components could be modified or replaced so long as wheelbase remained standard.

Brakes and operating system were free but components had to remain in the standard location.

Axle locating devices could not pass into the driver compartment; however, the rear seat well could be covered with sheet metal to satisfy this requirement.

Springs could be replaced by others of unrestricted origin, but with no modification of the number provided by the manufacturer.

The steering wheel and the front seats could be replaced, provided the seats of at least the same weight as the original ones be substituted to them.

Material of engine and luggage compartment covers, doors and fenders was free.

Fender extensions were allowed to cover the legal wheels and tires but should retain the standard opening shape as viewed from the side.

Any additional bodywork should not confuse the make and model identity of the car.

Bumpers and external decorative trim could be removed. Any substitute bumpers should have standard dimensions and shapes.

Cars should retain the dimensions provided by the manufacturer for his series-production.

All four wheels should have the same diameter.

Track dimension was limited by inner tire clearance and the permitted maximum car width.

The fitting of a stabilizer was authorized.