I recently came across the term “development mule” in an article at Jalopnik, and several Ford Mustang “mules” were featured in director David Gelb’s A Faster Horse, which documents the design, production, and development of the 2015 Mustang. I was fascinated with the term, so I did a little research. According to Wikipedia, a development mule, test mule, or (simply) mule in the automotive industry is,

a testbed vehicle equipped with prototype components requiring evaluation. They are often camouflaged to deceive competitors and thwart a curious automotive press.

So what is the difference between a “prototype” and a “development mule?”

Get on your mule and ride

The “GM Guide To Terms Used In Auto Body Design” defines a prototype as,

The original model during the evaluation stage in automotive engineering.

An automotive forum at eng-tips.com offers this definition for prototype,

A pre-mass production vehicle of new design that is used for testing and development purposes.

The following explanation from Wikipedia does a great job of defining the unique purpose and specific characteristic of an automotive development mule, test mule, or (simply) a mule:

Mules may also have advanced chassis and powertrain designs from a prospective vehicle that need testing, which can be effectively concealed in the body and interior of a similarly sized production model.

If no comparable vehicle is available in-house or an external benchmark is being used mules may be based on another manufacturer’s model. For example, in the 1970s the new powertrain package of first-generation Ford Fiesta was developed using mules based on the then class-leading Fiat 127, as Ford had no comparable compact model of similar size to utilize.

Mules are also used to conceal styling changes and visible telltales of performance alterations in near-production vehicles, receiving varying degrees of camouflage to deceive rival makers and thwart a curious automotive press. Such alterations can span from distracting shrinkwrap designs to substituting crude cylindric shapes for taillights, non-standard wheels, or assemblages of plastic and tape to hide a vehicle’s shape and design elements.

Perhaps the key to the meaning and use of the term mule in this context, is the vehicle’s purpose of “carrying” various “hybrid parts” for testing its “handling, roadloads, and powertrain” characteristics. After all, an actual Equus mulus by definition is a “hybrid” of a male donkey and a female horse. Mules are valued for their sure-footedness, strength, and endurance. Furthermore, they tend to be more independent than most domesticated equines, and can be effectively packed with various loads.

Etymology

Mule – I discovered several origin stories for the term at Stack Exchange:

In French or Italian, car racing teams were/are called “écurie (de course)” or “scuderia”, literally racing stable: the race cars are the horses and the replacement car is called in French “le mulet” (the mule) and in Italian “il muletto”.

Both in French and in English, the sense of mule/mulet later extended to development cars (testbed vehicle equipped with prototype components requiring evaluation).

Chevrolet’s practice car had fiberglass body, was called ‘the mule’ – 1956 Road & Track

…while the SS in both “Mule” and “show” variants ran they went like stink. The officially released lap time set by Fangio at Sebring in the prototype Mule was 3:27.2, a very respectable figure. – 1957 Car and Drive

With a rough fiberglass body this became the “Mule”, which went down to Sebring for on-the-spot trials while the actual race car was completed… the Mule was revised and cleaned up in detail to be exactly like the race SS, but the ax fell on the project before the ex-Mule could be assembled … this car, the Mule… – 1960 Car & Driver

…had built a pair of muletti — “mules” — whose design had been hastily roughed out by the same internal talent that had drawn up the Dischi Volanti and many other “house” designs. The workmanship of these muletti also was rough as they were never intended to be seen by the public. – 1964 Road and Track

In the comments section of the same post at Stack Exchange a reader contributed this observation,

My understanding is that a “mule” is a crude vehicle used to test engines and other components. Likely from its resemblance to a mechanical “mule” on a canal — basically a small locomotive with no cab, just frame, engine, and wheels. And that term, of course, comes from the animal it replaces. (I first read the term ca 1965. Likely it goes back at least 20 years. prior to that.

Charles Darwin, who knew a thing or two about the Origin of Species, wrote: “The mule always appears to me a most surprising animal. That a hybrid should possess more reason, memory, obstinacy, social affection, powers of muscular endurance, and length of life, than either of its parents, seems to indicate that art has here outdone nature.”

Automotive Industry Jargon: Bonus Glossary

Beltline: the line going from the hood which usually follows the bottom edge of the windows and continues to the trunk. The beltline is a major component of the vehicle’s overall appearance, as well as the safety aspect of blind spots.

Bubble Up: a preproduction stage of design. Also know as the theme stage or concept stage.

Brightwork: anything reflective added to a car to enhance appearance. May also be called chrome.

Buck: a full size model of a vehicle used to evaluate comfort, entrance, egress, vision; usually made of wood, metal, foam and/or fiberglass.

Clay Buck: a full size mock up of a vehicle made from a clay covered armature to show vehicle shape.

Down the Road Graphics: the styling of the front end of the car, which people will instantly recognize and associate with a manufacturer.

Concept Car: A Full size vehicle made to illustrate a design concept or idea. Usually with futuristic components and features; often shown at auto exhibitions and shows.

Greenhouse: the glassed-in upper section of the car’s body.

Oscar: a mannequin representing the 95th percentile male and used in packaging a vehicle.

Proveout Model: a clay model developed to verify surface drawing conformation with the appearance of the model originally approved by management from which a recorded fiberglass cast is subsequently made.

Show Car: a car having features or shapes not offered in production cars, and designed for display.

Trim Buck: a fullsize model showing interior design finishes of a specific model of automobile.

Tumblehome: refers to the way the sides of a car rounds inward toward the roof, specifically of the greenhouse above the beltline.

Tangents

A Faster Horse: the title for David Gelb’s documentary was derived from the adage “If I had asked people what they wanted, they would have said faster horses.” which is often attributed to Henry Ford. (For more on that story see our “Debunking Henry Ford’s ‘faster horse’ quote” blog post.

Giuseppe Mulè: was an Italian composer and conductor.

Headless Mule: is a character in Brazilian folklore. In most tales, it is the ghost of a woman that has been cursed by God for her sins. And condemned to turn into a fire-spewing headless mule, galloping through the countryside from Thursday’s sundown to Friday’s sunrise.

Hinny: is a domestic equine hybrid that is the offspring of a male horse, a stallion, and a female donkey, a jenny. It is the reciprocal cross to the more common mule, which is the product of a male donkey and a female horse.

Mule: is a small electric tractor used for hauling over short distances.

Spinning Mule: is a machine that makes thread or yarn from fibers.

Twenty-Mule Teams: were actually teams of eighteen mules and two horses attached to large wagons that ferried borax out of Death Valley, California.

Sources

• Development Mule – Wikipedia page
• Mule Car vs. Prototype – Automotive forum, eng-tips.com
• GM Guide To Terms Used In Auto Body Design (1991) – Courtesy of General Motors
• Mule – Wikipedia page
• What is the origin of mule in test mule? – English Language & Usage, Stack Exchange
• Glossary of Automotive Design – Wikipedia