Mexican overdrive

slang The act of coasting downhill in an automobile with the gears in neutral, with the aim of either saving fuel or making the vehicle go faster than would otherwise be possible. He rolled down the mountain in Mexican overdrive, but as he reached a bend in the road, the truck was going too fast, and it went careering through the guard rails and into the ravine.

Mexican overdrive

the neutral gear position used when coasting downhill. US informal

This expression originated in the mid 20th century, especially in language used by long-distance truck drivers.