Volvo P1800E Overdrive

OVERDRIVE is an auxiliary transmission used mainly in manual transmission models of the 1970s and 1980s.
Its essence is that it reduces the speed of the engine at high speeds, thus ensuring a quieter, more economical and less wear and tear operation. In practice, it's like the transmission gets half an extra gear - e.g. between 4th and 5th – which resulted in significant fuel savings on the highway.