automatic overdrive

Mid 1980's 904s came with a lockup converter (IIRC A998/A999?).

It's a compromise between overdrive and cutting the trans crossmember.

Gets rid of converter slip at road speed and makes a true 1:1 3rd gear.

Should get an MPG or 2.

...but I'd not drag race with one, although you can get performance lock up converters.