The act of coasting downhill in an automobile with the gears in neutral, with the aim of either saving on the fuel or to make the car go faster than would otherwise be possible. They rolled down the mountain in Mexican overdrive, but as they reached a bend in the road, they were going too fast to control the car, and it went careering through the guard rails and into the ravine.
Mexican overdrivethe 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.