A test mule is a prototype that is used for performance evaluation. It is a common term for preproduction cars, but is also widely used in non-automotive product development. Where did the term come from? I have asked product developers in several industries and they don't know. There are also pony engines and steam donkeys, and engines generally replaced horses for power production, but I can't find the origin of test mules. One possible origin is that a mule was used as direct comparison for evaluating the power of something such as a wind mill or water wheel. Records of tests of machinery vs animals go back at least a thousand years, but the test mule appears to be recent enough to hope for a first-person account.

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    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.)
    – Hot Licks
    Commented Apr 9, 2016 at 1:28
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    The earliest Google Books match I could scare up is from a 1971 issue of Automobile Quarterly: "When a test mule was built, its lack of torsional rigidity was immediately apparent. This led to vicious handling qualities and to breaking of welds, whose many repairs are still to be seen. So are the huge cast-iron weights added high in the rear-central area to improve the weight distribution of the nose-heavy chassis."
    – Sven Yargs
    Commented Apr 9, 2016 at 1:32
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    There are all sorts of equipment called mules. They are generally low-speed 'tugs' of some sort. A canal mule is a perfect example. The launcher closure door on a Minuteman silo is pumped open with a hydraulic mule. This is a bit easier to get my head around, as they appear to be a suitable substitute for an actual mule.
    – Phil Sweet
    Commented Apr 9, 2016 at 1:43
  • A canal mule can be seen in the picture accompanying this Wikipedia article. The one pictured is, of course, much more sophisticated than the ones of 50-100 years ago. Here is a shot of an older design. And about halfway down this article are some pictures of old (mostly electric) mules in Europe, in a wide variety of form factors.
    – Hot Licks
    Commented Apr 9, 2016 at 2:18
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    There is the possibility that "mule" as a test bed, spliced together from odd components, is playing on the hybrid (and somewhat "bastard") nature of the animal, being the offspring of a horse and a donkey, and sterile.
    – Hot Licks
    Commented Apr 9, 2016 at 2:44

2 Answers 2


It may have originated in Italian rather than English, because the 1964 Road and Track vol. 16, page 43 says:

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.

There are slightly earlier examples in English:

1962 Road and Track, Volume 14, page 124:

After this crash, the practice/testing "mule" was hauled out, the salvageable pieces from Gurney's previous mount installed. On the day following the crash, Dashing Dan qualified at 149.019-mph, what was ultimately to be 17th fastest.

1960 Car & Driver, Volume 6, page 46:

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...

1957 Car and Drive, volume 3, page 72

...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.

1956 Road & Track, volume 8, page 220

Chevrolet's practice car had fiberglass body, was called "the mule"

There is earlier literal use of "test mule", such as in the 1955 Journal of the American Veterinary Medical Association, Volume 126, page 285

When test mule 1434 and test horse 1121 were subsequently challenged with a known strain of virus, both promptly developed clinical symptoms of infectious anemia


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).

The italian Wikipedia article on muletto refers to a French origin of that Italian word, because in italian, the animal is a "mulo".

However, I have any other evidence to support a French/Italian origin of "mule" in the sense of evaluation prototype. Even if the word is quite common in French, I only found one occurence in the very view Google books on car racing: the book was written in 1958.

  • Thanks, that's a usage I didn't know about. Can you put a time frame on the usage of mule as a second car?
    – Phil Sweet
    Commented Apr 26, 2016 at 16:24
  • what about "muleta" as in "This size has been well proved in competition in the Targa Florio and Le Mans as well as for many road miles in the muleta car" 1961 books.google.com/…
    – DavePhD
    Commented Apr 26, 2016 at 17:11
  • @ DavePhD - I believe that in this text "muleta" is a typo.
    – Graffito
    Commented Apr 26, 2016 at 18:45
  • @Graffito the next page of the same reference says: Ferrari, as the home team, yielded to the organizers' pleas and mustered up five cars; four 120° machines for P. Hill, Trips, Richie Ginther and Giancarlo Baghetti plus one 60° "muleta" for Ricardo Rodriguez.
    – DavePhD
    Commented Apr 26, 2016 at 18:57
  • and the phrase "Lamborghini muleta" seems to be in current use, meaning Lamborghini concept car as far as I can tell, like here: carbodydesign.com/gallery/2009/11/…
    – DavePhD
    Commented Apr 26, 2016 at 19:47

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