Mexican-overdrive Definition

noun

(US, slang) The neutral gear when used while a vehicle is moving, especially when it is used to save fuel or to coast down hill faster than a vehicle can go in gear. (This is illegal in some places.)

He shifted his car out of gear, at the top of the hill, and coasted down in Mexican overdrive.

Wiktionary

Origin of Mexican-overdrive

Based on NASCAR teams' experience in the early 1950s in the legendary Carerra Panamerica where the long hills would have otherwise overrevved the engines

From Wiktionary