The American-English phrase Mexican overdrive, which originated in lorry-drivers’ slang, designates the practice of coasting downhill in a motor vehicle, with the engine disengaged.

This phrase occurs, for example, in the following from Wigwam Harry, by Jim Robb, published in the Whitehorse Star (Whitehorse, Yukon, Canada) of Friday 27^th November 2020:

Wigwam Harry, whose right name was Harry Fieck […] was born in 1900 in Stratford, Ontario.
Harry headed west in 1918 to become a cowhand around Swift Current, Saskatchewan. Then he went north to Churchill, Manitoba, to work as a crane operator.
He came to the Yukon in 1941 to work on the Alaska Highway and the Canol Road as a truck driver.
Once, a U.S. army captain asked Harry to drive him somewhere on the highway and, as Harry used to drive like a bat out of bell, he really scared the captain into saying, “Can’t you get this thing stopped?”
Harry, grinning his one-toothed grin, had slipped into ‘Mexican Overdrive’ (kicked the truck out of gear) going down Iron Creek Hill, a sort of suicide-type hill.

Mexican overdrive is one of several phrases in which the adjective Mexican is used to designate simple or basic devices or processes compared unfavourably with more advanced or sophisticated equivalents.—For example:

– In quotation 1, below, the phrase Mexican supercharger designates “an air-vented rig” (rig denotes an articulated lorry).

– In the following from Power in their hands, an article by A. S. Fry about Māori machine operators, published in the New-Zealand quarterly magazine Te Ao Hou / The New World of March 1961 (reprinted from the New Zealand Listener of Friday 21^st October 1960), the phrase Mexican side-loader designates a shovel:

“It’s interesting work—anything’s better than the banjo.”
“The banjo?”
“A shovel. Some people call it a Mexican side-loader.”

—Cf. also the phrases Mexican standoff and Montezuma’s Revenge.

The earliest occurrences of the phrase Mexican overdrive that I have found are as follows, in chronological order:

1-: From the column Western Round-Up, by Jim Joseph, published in the San Luis Obispo Telegram-Tribune (San Luis Obispo, California, USA) of Saturday 4^th June 1949:

POCATELLO, Idaho—A new vocabulary is growing up in the west, passed on town to town by line-drivers on big, long-haul diesels.
At every truck-stop cafe you hear new words—new terms coined during long hours behind a wheel, coined where truck drivers meet. Gradually they’re passing down through society—someday perhaps, into dictionaries as part of Americana.
This is a preview of words to come.
Basic for this new rhetoric are the truck types themselves. An International Truck is a “corn-binder”—“because it’s produced in the corn-belt states,” one driver explained. Just plain “binder” are brakes. But if you’re coming off a hill and the brakes start burning, you have “fired your binders.”
A General Motors truck—a GMC—is a “jimmy.” Just say GMC rapidly aloud and you’ll see how truck drivers coined that one.
An old truck is a “bucket of bolts”—probably as correct as it is descriptive.
Mixing Sticks
Coming off a hill (descending), Nick Nichols, who’s been driving big semis all his life, and who now works for Garrett Freight Lines, “mixed his sticks”—shifted gears.
These big diesels have 12 speeds forward, four speed on one gear, three on the other. By multiple combinations of these you get the 12 forward speeds. Hauling 40-tons up steep hills—like Utah hill near Cedar City—turns driver into gear shifting robots. Makes them “gear happy.” Sometimes drivers shift 150 times ascending Utah hill.
If a driver “boots out” going downhill—takes his truck out of gear—he’ll probably wreck or get fired. Once black-balled on a line drive, the dieselman had better start a new career.
Insurance spotters called “stool pigeons”—will tail a wayward truck for miles, report misdemeanors, and that usually means punching time clocks for the last time on a major line.
Another term for ungeared downhilling is “Mexican Overdrive”—a USAish vernacular with a misplaced Latin flavor . . . A “Mexican supercharger” is truckers’ lingo for an air-vented rig—which helps boost combustion on high altitude runs.
A trailer is a “box”; a truck and trailer, a “rig.”
Speed, More Speed
“Flying the bale” means plenty of speed. An old truck is not only a “bucket of bolts” but a “loser”—a rig that wrecks time schedules laboring up hills.
Few diesel trucks ever exceed 50 miles per hour. The reason: the Tachograph—sometimes called “stool pigeon”—Tachographs are automatic recorders, riding as silent partners—devices which record speed, distance traveled, time of stops. No speeding—no lingering with a “Tach” along—and no “booting out” on hills.
Actually Tachographs are a driver’s best friend. Let a tough cop slap a citation on a driver and the “Tach” more often than not stands up as chief witness for the defense.
Fooling “Tach”
But truck drivers can fool the tachograph—and cuss it all at the same time—in drivers’ lingo. One trucker clipped a nice young buck on a lonely mountain road. He stopped his “bucket of bolts,” skinned the deer, and highballed to the nearest town and a meat locker. How did he explain his 40-minute stop?
“Hell,” he told the road boss, “I was just fixin’ the motor.” And the Tachograph couldn’t prove otherwise.
Diesel-killed game is mighty tender they say—what with all the meat cubed—and the bones crushed. And it beats gun-hunting. 40 tons at 50 miles per hour packs more wallop than a 30-30.
Maybe they’ll coin a word for it—like “diesel meat”—and give English a long-haul flavor.

2-: From The Chico Enterprise-Record (Chico, California, USA) of Thursday 22^nd February 1951:

What About Those Yellow Buses?
Schoolkid Transportation Is Bigger Item Than You Think
By AL ALLEN

[…]
School buses must be cleaned after each day’s use. No driver may leave his bus while the engine is running or the brakes are released. Gasoline tanks must not be filled while the engine is running or there are pupils in the bus. Coasting with the clutch disengaged or the gears in neutral—the truck drivers’ “Mexican overdrive”—is illegal.