8T was originally designed for only a small amount of tape inside (less weight, smaller ratio of inside/outside diameter, less slip/compression). For pre-packaged single advertising content. DJ just jammed that cartridge/ad in the player. End of AD campaign, toss it in the trash quickly.Actually, that was the Fidelipac cartridge. It originally had two tracks on 1/4-inch tape, one monaural program audio and a second for cueing. Later stereo versions had either three tracks (two program and one cue) or two stereo tracks and a foil tab for cueing. These ran at 7.5 IPS for broadcasting. They weren't thrown away but bulk erased and reused. Unlike the 8-track, the pinch roller was part of the player, not integrated into the cartridge. When used with quality and well-maintained equipment they had decent fidelity. They needed special tape with the back side lubricated to allow slippage for the continuous loop. The typical size was the same as 8-track tapes for a commercial spot or popular top-40 song up to 10 minutes but larger carts were made that could play for an hour or more.
The identical mechanical cartridge format became the 4-track tape sold by Muntz Stereo-Pak as an add-on accessory for cars and each pair of stereo tracks held an album side. These, like the 8-track, ran at 3 3/4 IPS. They used a foil cueing tab for track change.
Automotive manufacturers and Lear came up with the 8-track to compete with the aftermarket 4-track. Same size cartridge but the pinch roller was built-in to the cartridge and tensioned by a V-shaped notch in the side of the cartridge. Thinner tracks resulted in poorer fidelity than the 4-track and the pinch roller setup was troublesome.